gdb: move displaced stepping logic to gdbarch, allow starting concurrent displaced steps

Today, GDB only allows a single displaced stepping operation to happen
per inferior at a time.  There is a single displaced stepping buffer per
inferior, whose address is fixed (obtained with
gdbarch_displaced_step_location), managed by infrun.c.

In the case of the AMD ROCm target [1] (in the context of which this
work has been done), it is typical to have thousands of threads (or
waves, in SMT terminology) executing the same code, hitting the same
breakpoint (possibly conditional) and needing to to displaced step it at
the same time.  The limitation of only one displaced step executing at a
any given time becomes a real bottleneck.

To fix this bottleneck, we want to make it possible for threads of a
same inferior to execute multiple displaced steps in parallel.  This
patch builds the foundation for that.

In essence, this patch moves the task of preparing a displaced step and
cleaning up after to gdbarch functions.  This allows using different
schemes for allocating and managing displaced stepping buffers for
different platforms.  The gdbarch decides how to assign a buffer to a
thread that needs to execute a displaced step.

On the ROCm target, we are able to allocate one displaced stepping
buffer per thread, so a thread will never have to wait to execute a
displaced step.

On Linux, the entry point of the executable if used as the displaced
stepping buffer, since we assume that this code won't get used after
startup.  From what I saw (I checked with a binary generated against
glibc and musl), on AMD64 we have enough space there to fit two
displaced stepping buffers.  A subsequent patch makes AMD64/Linux use
two buffers.

In addition to having multiple displaced stepping buffers, there is also
the idea of sharing displaced stepping buffers between threads.  Two
threads doing displaced steps for the same PC could use the same buffer
at the same time.  Two threads stepping over the same instruction (same
opcode) at two different PCs may also be able to share a displaced
stepping buffer.  This is an idea for future patches, but the
architecture built by this patch is made to allow this.

Now, the implementation details.  The main part of this patch is moving
the responsibility of preparing and finishing a displaced step to the
gdbarch.  Before this patch, preparing a displaced step is driven by the
displaced_step_prepare_throw function.  It does some calls to the
gdbarch to do some low-level operations, but the high-level logic is
there.  The steps are roughly:

- Ask the gdbarch for the displaced step buffer location
- Save the existing bytes in the displaced step buffer
- Ask the gdbarch to copy the instruction into the displaced step buffer
- Set the pc of the thread to the beginning of the displaced step buffer

Similarly, the "fixup" phase, executed after the instruction was
successfully single-stepped, is driven by the infrun code (function
displaced_step_finish).  The steps are roughly:

- Restore the original bytes in the displaced stepping buffer
- Ask the gdbarch to fixup the instruction result (adjust the target's
  registers or memory to do as if the instruction had been executed in
  its original location)

The displaced_step_inferior_state::step_thread field indicates which
thread (if any) is currently using the displaced stepping buffer, so it
is used by displaced_step_prepare_throw to check if the displaced
stepping buffer is free to use or not.

This patch defers the whole task of preparing and cleaning up after a
displaced step to the gdbarch.  Two new main gdbarch methods are added,
with the following semantics:

  - gdbarch_displaced_step_prepare: Prepare for the given thread to
    execute a displaced step of the instruction located at its current PC.
    Upon return, everything should be ready for GDB to resume the thread
    (with either a single step or continue, as indicated by
    gdbarch_displaced_step_hw_singlestep) to make it displaced step the
    instruction.

  - gdbarch_displaced_step_finish: Called when the thread stopped after
    having started a displaced step.  Verify if the instruction was
    executed, if so apply any fixup required to compensate for the fact
    that the instruction was executed at a different place than its
    original pc.  Release any resources that were allocated for this
    displaced step.  Upon return, everything should be ready for GDB to
    resume the thread in its "normal" code path.

The displaced_step_prepare_throw function now pretty much just offloads
to gdbarch_displaced_step_prepare and the displaced_step_finish function
offloads to gdbarch_displaced_step_finish.

The gdbarch_displaced_step_location method is now unnecessary, so is
removed.  Indeed, the core of GDB doesn't know how many displaced step
buffers there are nor where they are.

To keep the existing behavior for existing architectures, the logic that
was previously implemented in infrun.c for preparing and finishing a
displaced step is moved to displaced-stepping.c, to the
displaced_step_buffer class.  Architectures are modified to implement
the new gdbarch methods using this class.  The behavior is not expected
to change.

The other important change (which arises from the above) is that the
core of GDB no longer prevents concurrent displaced steps.  Before this
patch, start_step_over walks the global step over chain and tries to
initiate a step over (whether it is in-line or displaced).  It follows
these rules:

  - if an in-line step is in progress (in any inferior), don't start any
    other step over
  - if a displaced step is in progress for an inferior, don't start
    another displaced step for that inferior

After starting a displaced step for a given inferior, it won't start
another displaced step for that inferior.

In the new code, start_step_over simply tries to initiate step overs for
all the threads in the list.  But because threads may be added back to
the global list as it iterates the global list, trying to initiate step
overs, start_step_over now starts by stealing the global queue into a
local queue and iterates on the local queue.  In the typical case, each
thread will either:

  - have initiated a displaced step and be resumed
  - have been added back by the global step over queue by
    displaced_step_prepare_throw, because the gdbarch will have returned
    that there aren't enough resources (i.e. buffers) to initiate a
    displaced step for that thread

Lastly, if start_step_over initiates an in-line step, it stops
iterating, and moves back whatever remaining threads it had in its local
step over queue to the global step over queue.

Two other gdbarch methods are added, to handle some slightly annoying
corner cases.  They feel awkwardly specific to these cases, but I don't
see any way around them:

  - gdbarch_displaced_step_copy_insn_closure_by_addr: in
    arm_pc_is_thumb, arm-tdep.c wants to get the closure for a given
    buffer address.

  - gdbarch_displaced_step_restore_all_in_ptid: when a process forks
    (at least on Linux), the address space is copied.  If some displaced
    step buffers were in use at the time of the fork, we need to restore
    the original bytes in the child's address space.

These two adjustments are also made in infrun.c:

  - prepare_for_detach: there may be multiple threads doing displaced
    steps when we detach, so wait until all of them are done

  - handle_inferior_event: when we handle a fork event for a given
    thread, it's possible that other threads are doing a displaced step at
    the same time.  Make sure to restore the displaced step buffer
    contents in the child for them.

[1] https://github.com/ROCm-Developer-Tools/ROCgdb

gdb/ChangeLog:

	* displaced-stepping.h (struct
	displaced_step_copy_insn_closure): Adjust comments.
	(struct displaced_step_inferior_state) <step_thread,
	step_gdbarch, step_closure, step_original, step_copy,
	step_saved_copy>: Remove fields.
	(struct displaced_step_thread_state): New.
	(struct displaced_step_buffer): New.
	* displaced-stepping.c (displaced_step_buffer::prepare): New.
	(write_memory_ptid): Move from infrun.c.
	(displaced_step_instruction_executed_successfully): New,
	factored out of displaced_step_finish.
	(displaced_step_buffer::finish): New.
	(displaced_step_buffer::copy_insn_closure_by_addr): New.
	(displaced_step_buffer::restore_in_ptid): New.
	* gdbarch.sh (displaced_step_location): Remove.
	(displaced_step_prepare, displaced_step_finish,
	displaced_step_copy_insn_closure_by_addr,
	displaced_step_restore_all_in_ptid): New.
	* gdbarch.c: Re-generate.
	* gdbarch.h: Re-generate.
	* gdbthread.h (class thread_info) <displaced_step_state>: New
	field.
	(thread_step_over_chain_remove): New declaration.
	(thread_step_over_chain_next): New declaration.
	(thread_step_over_chain_length): New declaration.
	* thread.c (thread_step_over_chain_remove): Make non-static.
	(thread_step_over_chain_next): New.
	(global_thread_step_over_chain_next): Use
	thread_step_over_chain_next.
	(thread_step_over_chain_length): New.
	(global_thread_step_over_chain_enqueue): Add debug print.
	(global_thread_step_over_chain_remove): Add debug print.
	* infrun.h (get_displaced_step_copy_insn_closure_by_addr):
	Remove.
	* infrun.c (get_displaced_stepping_state): New.
	(displaced_step_in_progress_any_inferior): Remove.
	(displaced_step_in_progress_thread): Adjust.
	(displaced_step_in_progress): Adjust.
	(displaced_step_in_progress_any_thread): New.
	(get_displaced_step_copy_insn_closure_by_addr): Remove.
	(gdbarch_supports_displaced_stepping): Use
	gdbarch_displaced_step_prepare_p.
	(displaced_step_reset): Change parameter from inferior to
	thread.
	(displaced_step_prepare_throw): Implement using
	gdbarch_displaced_step_prepare.
	(write_memory_ptid): Move to displaced-step.c.
	(displaced_step_restore): Remove.
	(displaced_step_finish): Implement using
	gdbarch_displaced_step_finish.
	(start_step_over): Allow starting more than one displaced step.
	(prepare_for_detach): Handle possibly multiple threads doing
	displaced steps.
	(handle_inferior_event): Handle possibility that fork event
	happens while another thread displaced steps.
	* linux-tdep.h (linux_displaced_step_prepare): New.
	(linux_displaced_step_finish): New.
	(linux_displaced_step_copy_insn_closure_by_addr): New.
	(linux_displaced_step_restore_all_in_ptid): New.
	(linux_init_abi): Add supports_displaced_step parameter.
	* linux-tdep.c (struct linux_info) <disp_step_buf>: New field.
	(linux_displaced_step_prepare): New.
	(linux_displaced_step_finish): New.
	(linux_displaced_step_copy_insn_closure_by_addr): New.
	(linux_displaced_step_restore_all_in_ptid): New.
	(linux_init_abi): Add supports_displaced_step parameter,
	register displaced step methods if true.
	(_initialize_linux_tdep): Register inferior_execd observer.
	* amd64-linux-tdep.c (amd64_linux_init_abi_common): Add
	supports_displaced_step parameter, adjust call to
	linux_init_abi.  Remove call to
	set_gdbarch_displaced_step_location.
	(amd64_linux_init_abi): Adjust call to
	amd64_linux_init_abi_common.
	(amd64_x32_linux_init_abi): Likewise.
	* aarch64-linux-tdep.c (aarch64_linux_init_abi): Adjust call to
	linux_init_abi.  Remove call to
	set_gdbarch_displaced_step_location.
	* arm-linux-tdep.c (arm_linux_init_abi): Likewise.
	* i386-linux-tdep.c (i386_linux_init_abi): Likewise.
	* alpha-linux-tdep.c (alpha_linux_init_abi): Adjust call to
	linux_init_abi.
	* arc-linux-tdep.c (arc_linux_init_osabi): Likewise.
	* bfin-linux-tdep.c (bfin_linux_init_abi): Likewise.
	* cris-linux-tdep.c (cris_linux_init_abi): Likewise.
	* csky-linux-tdep.c (csky_linux_init_abi): Likewise.
	* frv-linux-tdep.c (frv_linux_init_abi): Likewise.
	* hppa-linux-tdep.c (hppa_linux_init_abi): Likewise.
	* ia64-linux-tdep.c (ia64_linux_init_abi): Likewise.
	* m32r-linux-tdep.c (m32r_linux_init_abi): Likewise.
	* m68k-linux-tdep.c (m68k_linux_init_abi): Likewise.
	* microblaze-linux-tdep.c (microblaze_linux_init_abi): Likewise.
	* mips-linux-tdep.c (mips_linux_init_abi): Likewise.
	* mn10300-linux-tdep.c (am33_linux_init_osabi): Likewise.
	* nios2-linux-tdep.c (nios2_linux_init_abi): Likewise.
	* or1k-linux-tdep.c (or1k_linux_init_abi): Likewise.
	* riscv-linux-tdep.c (riscv_linux_init_abi): Likewise.
	* s390-linux-tdep.c (s390_linux_init_abi_any): Likewise.
	* sh-linux-tdep.c (sh_linux_init_abi): Likewise.
	* sparc-linux-tdep.c (sparc32_linux_init_abi): Likewise.
	* sparc64-linux-tdep.c (sparc64_linux_init_abi): Likewise.
	* tic6x-linux-tdep.c (tic6x_uclinux_init_abi): Likewise.
	* tilegx-linux-tdep.c (tilegx_linux_init_abi): Likewise.
	* xtensa-linux-tdep.c (xtensa_linux_init_abi): Likewise.
	* ppc-linux-tdep.c (ppc_linux_init_abi): Adjust call to
	linux_init_abi.  Remove call to
	set_gdbarch_displaced_step_location.
	* arm-tdep.c (arm_pc_is_thumb): Call
	gdbarch_displaced_step_copy_insn_closure_by_addr instead of
	get_displaced_step_copy_insn_closure_by_addr.
	* rs6000-aix-tdep.c (rs6000_aix_init_osabi): Adjust calls to
	clear gdbarch methods.
	* rs6000-tdep.c (struct ppc_inferior_data): New structure.
	(get_ppc_per_inferior): New function.
	(ppc_displaced_step_prepare): New function.
	(ppc_displaced_step_finish): New function.
	(ppc_displaced_step_restore_all_in_ptid): New function.
	(rs6000_gdbarch_init): Register new gdbarch methods.
	* s390-tdep.c (s390_gdbarch_init): Don't call
	set_gdbarch_displaced_step_location, set new gdbarch methods.

gdb/testsuite/ChangeLog:

	* gdb.arch/amd64-disp-step-avx.exp: Adjust pattern.
	* gdb.threads/forking-threads-plus-breakpoint.exp: Likewise.
	* gdb.threads/non-stop-fair-events.exp: Likewise.

Change-Id: I387cd235a442d0620ec43608fd3dc0097fcbf8c8

1 files changed, 143 insertions, 192 deletions

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 9ac75ef..e7b69ae 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -1452,21 +1452,6 @@ step_over_info_valid_p (void)
    displaced step operation on it.  See displaced_step_prepare and
    displaced_step_finish for details.  */
 
-/* Returns true if any inferior has a thread doing a displaced
-   step.  */
-
-static bool
-displaced_step_in_progress_any_inferior ()
-{
-  for (inferior *i : all_inferiors ())
-    {
-      if (i->displaced_step_state.step_thread != nullptr)
-	return true;
-    }
-
-  return false;
-}
-
 /* Return true if THREAD is doing a displaced step.  */
 
 static bool
@@ -1474,7 +1459,7 @@ displaced_step_in_progress_thread (thread_info *thread)
 {
   gdb_assert (thread != NULL);
 
-  return thread->inf->displaced_step_state.step_thread == thread;
+  return thread->displaced_step_state.in_progress ();
 }
 
 /* Return true if INF has a thread doing a displaced step.  */
@@ -1482,25 +1467,21 @@ displaced_step_in_progress_thread (thread_info *thread)
 static bool
 displaced_step_in_progress (inferior *inf)
 {
-  return inf->displaced_step_state.step_thread != nullptr;
+  return inf->displaced_step_state.in_progress_count > 0;
 }
 
-/* If inferior is in displaced stepping, and ADDR equals to starting address
-   of copy area, return corresponding displaced_step_copy_insn_closure.
-   Otherwise, return NULL.  */
+/* Return true if any thread is doing a displaced step.  */
 
-displaced_step_copy_insn_closure *
-get_displaced_step_copy_insn_closure_by_addr (CORE_ADDR addr)
+static bool
+displaced_step_in_progress_any_thread ()
 {
-  displaced_step_inferior_state &displaced
-    = current_inferior ()->displaced_step_state;
-
-  /* If checking the mode of displaced instruction in copy area.  */
-  if (displaced.step_thread != nullptr
-      && displaced.step_copy == addr)
-    return displaced.step_closure.get ();
+  for (inferior *inf : all_non_exited_inferiors ())
+    {
+      if (displaced_step_in_progress (inf))
+	return true;
+    }
 
-  return NULL;
+  return false;
 }
 
 static void
@@ -1512,12 +1493,15 @@ infrun_inferior_exit (struct inferior *inf)
 static void
 infrun_inferior_execd (inferior *inf)
 {
-  /* If a thread was doing a displaced step in this inferior at the moment of
-     the exec, it no longer exists.  Even if the exec'ing thread was the one
+  /* If some threads where was doing a displaced step in this inferior at the
+     moment of the exec, they no longer exist.  Even if the exec'ing thread
      doing a displaced step, we don't want to to any fixup nor restore displaced
      stepping buffer bytes.  */
   inf->displaced_step_state.reset ();
 
+  for (thread_info *thread : inf->threads ())
+    thread->displaced_step_state.reset ();
+
   /* Since an in-line step is done with everything else stopped, if there was
      one in progress at the time of the exec, it must have been the exec'ing
      thread.  */
@@ -1555,9 +1539,9 @@ show_can_use_displaced_stepping (struct ui_file *file, int from_tty,
 static bool
 gdbarch_supports_displaced_stepping (gdbarch *arch)
 {
-  /* Only check for the presence of step_copy_insn.  Other required methods
-     are checked by the gdbarch validation.  */
-  return gdbarch_displaced_step_copy_insn_p (arch);
+  /* Only check for the presence of `prepare`.  The gdbarch verification ensures
+     that if `prepare` is provided, so is `finish`.  */
+  return gdbarch_displaced_step_prepare_p (arch);
 }
 
 /* Return non-zero if displaced stepping can/should be used to step
@@ -1595,10 +1579,10 @@ use_displaced_stepping (thread_info *tp)
   return true;
 }
 
-/* Simple function wrapper around displaced_step_inferior_state::reset.  */
+/* Simple function wrapper around displaced_step_thread_state::reset.  */
 
 static void
-displaced_step_reset (displaced_step_inferior_state *displaced)
+displaced_step_reset (displaced_step_thread_state *displaced)
 {
   displaced->reset ();
 }
@@ -1649,10 +1633,8 @@ displaced_step_prepare_throw (thread_info *tp)
 {
   regcache *regcache = get_thread_regcache (tp);
   struct gdbarch *gdbarch = regcache->arch ();
-  const address_space *aspace = regcache->aspace ();
-  CORE_ADDR original, copy;
-  ULONGEST len;
-  int status;
+  displaced_step_thread_state &disp_step_thread_state
+    = tp->displaced_step_state;
 
   /* We should never reach this function if the architecture does not
      support displaced stepping.  */
@@ -1667,15 +1649,14 @@ displaced_step_prepare_throw (thread_info *tp)
      jump/branch).  */
   tp->control.may_range_step = 0;
 
-  /* We have to displaced step one thread at a time, as we only have
-     access to a single scratch space per inferior.  */
-
-  displaced_step_inferior_state *displaced = &tp->inf->displaced_step_state;
+  /* We are about to start a displaced step for this thread.  If one is already
+     in progress, something's wrong.  */
+  gdb_assert (!disp_step_thread_state.in_progress ());
 
-  if (displaced->step_thread != nullptr)
+  if (tp->inf->displaced_step_state.unavailable)
     {
-      /* Already waiting for a displaced step to finish.  Defer this
-	 request and place in queue.  */
+      /* The gdbarch tells us it's not worth asking to try a prepare because
+	 it is likely that it will return unavailable, so don't bother asking.  */
 
       displaced_debug_printf ("deferring step of %s",
 			      target_pid_to_str (tp->ptid).c_str ());
@@ -1683,79 +1664,54 @@ displaced_step_prepare_throw (thread_info *tp)
       global_thread_step_over_chain_enqueue (tp);
       return DISPLACED_STEP_PREPARE_STATUS_UNAVAILABLE;
     }
-  else
-    displaced_debug_printf ("stepping %s now",
-			    target_pid_to_str (tp->ptid).c_str ());
 
-  displaced_step_reset (displaced);
+  displaced_debug_printf ("displaced-stepping %s now",
+			  target_pid_to_str (tp->ptid).c_str ());
 
   scoped_restore_current_thread restore_thread;
 
   switch_to_thread (tp);
 
-  original = regcache_read_pc (regcache);
+  CORE_ADDR original_pc = regcache_read_pc (regcache);
+  CORE_ADDR displaced_pc;
 
-  copy = gdbarch_displaced_step_location (gdbarch);
-  len = gdbarch_max_insn_length (gdbarch);
+  displaced_step_prepare_status status
+    = gdbarch_displaced_step_prepare (gdbarch, tp, displaced_pc);
 
-  if (breakpoint_in_range_p (aspace, copy, len))
+  if (status == DISPLACED_STEP_PREPARE_STATUS_CANT)
     {
-      /* There's a breakpoint set in the scratch pad location range
-	 (which is usually around the entry point).  We'd either
-	 install it before resuming, which would overwrite/corrupt the
-	 scratch pad, or if it was already inserted, this displaced
-	 step would overwrite it.  The latter is OK in the sense that
-	 we already assume that no thread is going to execute the code
-	 in the scratch pad range (after initial startup) anyway, but
-	 the former is unacceptable.  Simply punt and fallback to
-	 stepping over this breakpoint in-line.  */
-      displaced_debug_printf ("breakpoint set in scratch pad.  "
-			      "Stepping over breakpoint in-line instead.");
+      displaced_debug_printf ("failed to prepare (%s)",
+			      target_pid_to_str (tp->ptid).c_str ());
 
       return DISPLACED_STEP_PREPARE_STATUS_CANT;
     }
+  else if (status == DISPLACED_STEP_PREPARE_STATUS_UNAVAILABLE)
+    {
+      /* Not enough displaced stepping resources available, defer this
+	 request by placing it the queue.  */
 
-  /* Save the original contents of the copy area.  */
-  displaced->step_saved_copy.resize (len);
-  status = target_read_memory (copy, displaced->step_saved_copy.data (), len);
-  if (status != 0)
-    throw_error (MEMORY_ERROR,
-		 _("Error accessing memory address %s (%s) for "
-		   "displaced-stepping scratch space."),
-		 paddress (gdbarch, copy), safe_strerror (status));
+      displaced_debug_printf ("not enough resources available, "
+			      "deferring step of %s",
+			      target_pid_to_str (tp->ptid).c_str ());
 
-  displaced_debug_printf ("saved %s: %s",
-			  paddress (gdbarch, copy),
-			  displaced_step_dump_bytes
-			    (displaced->step_saved_copy.data (), len).c_str ());
+      global_thread_step_over_chain_enqueue (tp);
 
-  displaced->step_closure
-    = gdbarch_displaced_step_copy_insn (gdbarch, original, copy, regcache);
-  if (displaced->step_closure == NULL)
-    {
-      /* The architecture doesn't know how or want to displaced step
-	 this instruction or instruction sequence.  Fallback to
-	 stepping over the breakpoint in-line.  */
-      return DISPLACED_STEP_PREPARE_STATUS_CANT;
+      return DISPLACED_STEP_PREPARE_STATUS_UNAVAILABLE;
     }
 
+  gdb_assert (status == DISPLACED_STEP_PREPARE_STATUS_OK);
+
   /* Save the information we need to fix things up if the step
      succeeds.  */
-  displaced->step_thread = tp;
-  displaced->step_gdbarch = gdbarch;
-  displaced->step_original = original;
-  displaced->step_copy = copy;
-
-  {
-    displaced_step_reset_cleanup cleanup (displaced);
+  disp_step_thread_state.set (gdbarch);
 
-    /* Resume execution at the copy.  */
-    regcache_write_pc (regcache, copy);
+  tp->inf->displaced_step_state.in_progress_count++;
 
-    cleanup.release ();
-  }
-
-  displaced_debug_printf ("displaced pc to %s", paddress (gdbarch, copy));
+  displaced_debug_printf ("prepared successfully thread=%s, "
+			  "original_pc=%s, displaced_pc=%s",
+			  target_pid_to_str (tp->ptid).c_str (),
+			  paddress (gdbarch, original_pc),
+			  paddress (gdbarch, displaced_pc));
 
   return DISPLACED_STEP_PREPARE_STATUS_OK;
 }
@@ -1797,33 +1753,6 @@ displaced_step_prepare (thread_info *thread)
   return status;
 }
 
-static void
-write_memory_ptid (ptid_t ptid, CORE_ADDR memaddr,
-		   const gdb_byte *myaddr, int len)
-{
-  scoped_restore save_inferior_ptid = make_scoped_restore (&inferior_ptid);
-
-  inferior_ptid = ptid;
-  write_memory (memaddr, myaddr, len);
-}
-
-/* Restore the contents of the copy area for thread PTID.  */
-
-static void
-displaced_step_restore (struct displaced_step_inferior_state *displaced,
-			ptid_t ptid)
-{
-  ULONGEST len = gdbarch_max_insn_length (displaced->step_gdbarch);
-
-  write_memory_ptid (ptid, displaced->step_copy,
-		     displaced->step_saved_copy.data (), len);
-
-  displaced_debug_printf ("restored %s %s",
-			  target_pid_to_str (ptid).c_str (),
-			  paddress (displaced->step_gdbarch,
-				    displaced->step_copy));
-}
-
 /* If we displaced stepped an instruction successfully, adjust registers and
    memory to yield the same effect the instruction would have had if we had
    executed it at its original address, and return
@@ -1836,13 +1765,15 @@ displaced_step_restore (struct displaced_step_inferior_state *displaced,
 static displaced_step_finish_status
 displaced_step_finish (thread_info *event_thread, enum gdb_signal signal)
 {
-  displaced_step_inferior_state *displaced
-    = &event_thread->inf->displaced_step_state;
+  displaced_step_thread_state *displaced = &event_thread->displaced_step_state;
 
-  /* Was this event for the thread we displaced?  */
-  if (displaced->step_thread != event_thread)
+  /* Was this thread performing a displaced step?  */
+  if (!displaced->in_progress ())
     return DISPLACED_STEP_FINISH_STATUS_OK;
 
+  gdb_assert (event_thread->inf->displaced_step_state.in_progress_count > 0);
+  event_thread->inf->displaced_step_state.in_progress_count--;
+
   /* Fixup may need to read memory/registers.  Switch to the thread
      that we're fixing up.  Also, target_stopped_by_watchpoint checks
      the current thread, and displaced_step_restore performs ptid-dependent
@@ -1851,35 +1782,10 @@ displaced_step_finish (thread_info *event_thread, enum gdb_signal signal)
 
   displaced_step_reset_cleanup cleanup (displaced);
 
-  displaced_step_restore (displaced, displaced->step_thread->ptid);
-
-  /* Did the instruction complete successfully?  */
-  if (signal == GDB_SIGNAL_TRAP
-      && !(target_stopped_by_watchpoint ()
-	   && (gdbarch_have_nonsteppable_watchpoint (displaced->step_gdbarch)
-	       || target_have_steppable_watchpoint ())))
-    {
-      /* Fix up the resulting state.  */
-      gdbarch_displaced_step_fixup (displaced->step_gdbarch,
-				    displaced->step_closure.get (),
-				    displaced->step_original,
-				    displaced->step_copy,
-				    get_thread_regcache (displaced->step_thread));
-
-      return DISPLACED_STEP_FINISH_STATUS_OK;
-    }
-  else
-    {
-      /* Since the instruction didn't complete, all we can do is
-	 relocate the PC.  */
-      struct regcache *regcache = get_thread_regcache (event_thread);
-      CORE_ADDR pc = regcache_read_pc (regcache);
-
-      pc = displaced->step_original + (pc - displaced->step_copy);
-      regcache_write_pc (regcache, pc);
-
-      return DISPLACED_STEP_FINISH_STATUS_NOT_EXECUTED;
-    }
+  /* Do the fixup, and release the resources acquired to do the displaced
+     step. */
+  return gdbarch_displaced_step_finish (displaced->get_original_gdbarch (),
+					event_thread, signal);
 }
 
 /* Data to be passed around while handling an event.  This data is
@@ -1927,14 +1833,42 @@ static step_over_what thread_still_needs_step_over (struct thread_info *tp);
 static bool
 start_step_over (void)
 {
-  struct thread_info *tp, *next;
+  thread_info *next;
 
   /* Don't start a new step-over if we already have an in-line
      step-over operation ongoing.  */
   if (step_over_info_valid_p ())
     return false;
 
-  for (tp = global_thread_step_over_chain_head; tp != NULL; tp = next)
+  /* Steal the global thread step over chain.  As we try to initiate displaced
+     steps, threads will be enqueued in the global chain if no buffers are
+     available.  If we iterated on the global chain directly, we might iterate
+     indefinitely.  */
+  thread_info *threads_to_step = global_thread_step_over_chain_head;
+  global_thread_step_over_chain_head = NULL;
+
+  infrun_debug_printf ("stealing global queue of threads to step, length = %d",
+		       thread_step_over_chain_length (threads_to_step));
+
+  bool started = false;
+
+  /* On scope exit (whatever the reason, return or exception), if there are
+     threads left in the THREADS_TO_STEP chain, put back these threads in the
+     global list.  */
+  SCOPE_EXIT
+    {
+      if (threads_to_step == nullptr)
+	infrun_debug_printf ("step-over queue now empty");
+      else
+	{
+	  infrun_debug_printf ("putting back %d threads to step in global queue",
+			       thread_step_over_chain_length (threads_to_step));
+
+	  global_thread_step_over_chain_enqueue_chain (threads_to_step);
+	}
+    };
+
+  for (thread_info *tp = threads_to_step; tp != NULL; tp = next)
     {
       struct execution_control_state ecss;
       struct execution_control_state *ecs = &ecss;
@@ -1943,12 +1877,23 @@ start_step_over (void)
 
       gdb_assert (!tp->stop_requested);
 
-      next = global_thread_step_over_chain_next (tp);
+      next = thread_step_over_chain_next (threads_to_step, tp);
 
-      /* If this inferior already has a displaced step in process,
-	 don't start a new one.  */
-      if (displaced_step_in_progress (tp->inf))
-	continue;
+      if (tp->inf->displaced_step_state.unavailable)
+	{
+	  /* The arch told us to not even try preparing another displaced step
+	     for this inferior.  Just leave the thread in THREADS_TO_STEP, it
+	     will get moved to the global chain on scope exit.  */
+	  continue;
+	}
+
+      /* Remove thread from the THREADS_TO_STEP chain.  If anything goes wrong
+	 while we try to prepare the displaced step, we don't add it back to
+	 the global step over chain.  This is to avoid a thread staying in the
+	 step over chain indefinitely if something goes wrong when resuming it
+	 If the error is intermittent and it still needs a step over, it will
+	 get enqueued again when we try to resume it normally.  */
+      thread_step_over_chain_remove (&threads_to_step, tp);
 
       step_what = thread_still_needs_step_over (tp);
       must_be_in_line = ((step_what & STEP_OVER_WATCHPOINT)
@@ -1958,13 +1903,11 @@ start_step_over (void)
       /* We currently stop all threads of all processes to step-over
 	 in-line.  If we need to start a new in-line step-over, let
 	 any pending displaced steps finish first.  */
-      if (must_be_in_line && displaced_step_in_progress_any_inferior ())
-	return false;
-
-      global_thread_step_over_chain_remove (tp);
-
-      if (global_thread_step_over_chain_head == NULL)
-	infrun_debug_printf ("step-over queue now empty");
+      if (must_be_in_line && displaced_step_in_progress_any_thread ())
+	{
+	  global_thread_step_over_chain_enqueue (tp);
+	  continue;
+	}
 
       if (tp->control.trap_expected
 	  || tp->resumed
@@ -1998,13 +1941,27 @@ start_step_over (void)
       if (!ecs->wait_some_more)
 	error (_("Command aborted."));
 
-      gdb_assert (tp->resumed);
+      /* If the thread's step over could not be initiated because no buffers
+	 were available, it was re-added to the global step over chain.  */
+      if (tp->resumed)
+	{
+	  infrun_debug_printf ("[%s] was resumed.",
+			       target_pid_to_str (tp->ptid).c_str ());
+	  gdb_assert (!thread_is_in_step_over_chain (tp));
+	}
+      else
+	{
+	  infrun_debug_printf ("[%s] was NOT resumed.",
+			       target_pid_to_str (tp->ptid).c_str ());
+	  gdb_assert (thread_is_in_step_over_chain (tp));
+	}
 
       /* If we started a new in-line step-over, we're done.  */
       if (step_over_info_valid_p ())
 	{
 	  gdb_assert (tp->control.trap_expected);
-	  return true;
+	  started = true;
+	  break;
 	}
 
       if (!target_is_non_stop_p ())
@@ -2017,7 +1974,8 @@ start_step_over (void)
 	  /* With remote targets (at least), in all-stop, we can't
 	     issue any further remote commands until the program stops
 	     again.  */
-	  return true;
+	  started = true;
+	  break;
 	}
 
       /* Either the thread no longer needed a step-over, or a new
@@ -2026,7 +1984,7 @@ start_step_over (void)
 	 displaced step on a thread of other process. */
     }
 
-  return false;
+  return started;
 }
 
 /* Update global variables holding ptids to hold NEW_PTID if they were
@@ -3611,18 +3569,16 @@ prepare_for_detach (void)
   struct inferior *inf = current_inferior ();
   ptid_t pid_ptid = ptid_t (inf->pid);
 
-  displaced_step_inferior_state *displaced = &inf->displaced_step_state;
-
   /* Is any thread of this process displaced stepping?  If not,
      there's nothing else to do.  */
-  if (displaced->step_thread == nullptr)
+  if (displaced_step_in_progress (inf))
     return;
 
   infrun_debug_printf ("displaced-stepping in-process while detaching");
 
   scoped_restore restore_detaching = make_scoped_restore (&inf->detaching, true);
 
-  while (displaced->step_thread != nullptr)
+  while (displaced_step_in_progress (inf))
     {
       struct execution_control_state ecss;
       struct execution_control_state *ecs;
@@ -5288,17 +5244,12 @@ handle_inferior_event (struct execution_control_state *ecs)
 	struct gdbarch *gdbarch = regcache->arch ();
 	inferior *parent_inf = find_inferior_ptid (ecs->target, ecs->ptid);
 
-	/* If this is a fork (child gets its own address space copy) and the
-	   displaced step buffer was in use at the time of the fork, restore
-	   displaced step buffer bytes in the child process.  */
+	/* If this is a fork (child gets its own address space copy) and some
+	   displaced step buffers were in use at the time of the fork, restore
+	   the displaced step buffer bytes in the child process.  */
 	if (ecs->ws.kind == TARGET_WAITKIND_FORKED)
-	  {
-	    displaced_step_inferior_state *displaced
-	      = &parent_inf->displaced_step_state;
-
-	    if (displaced->step_thread != nullptr)
-	      displaced_step_restore (displaced, ecs->ws.value.related_pid);
-	  }
+	  gdbarch_displaced_step_restore_all_in_ptid
+	    (gdbarch, parent_inf, ecs->ws.value.related_pid);
 
 	/* If displaced stepping is supported, and thread ecs->ptid is
 	   displaced stepping.  */