diff options
Diffstat (limited to 'nptl/pthread_create.c')
| -rw-r--r-- | nptl/pthread_create.c | 207 |
1 files changed, 174 insertions, 33 deletions
diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c index 867e347..2ef2bcb 100644 --- a/nptl/pthread_create.c +++ b/nptl/pthread_create.c @@ -54,25 +54,141 @@ unsigned int __nptl_nthreads = 1; /* Code to allocate and deallocate a stack. */ #include "allocatestack.c" -/* createthread.c defines this function, and two macros: +/* CONCURRENCY NOTES: + + Understanding who is the owner of the 'struct pthread' or 'PD' + (refers to the value of the 'struct pthread *pd' function argument) + is critically important in determining exactly which operations are + allowed and which are not and when, particularly when it comes to the + implementation of pthread_create, pthread_join, pthread_detach, and + other functions which all operate on PD. + + The owner of PD is responsible for freeing the final resources + associated with PD, and may examine the memory underlying PD at any + point in time until it frees it back to the OS or to reuse by the + runtime. + + The thread which calls pthread_create is called the creating thread. + The creating thread begins as the owner of PD. + + During startup the new thread may examine PD in coordination with the + owner thread (which may be itself). + + The four cases of ownership transfer are: + + (1) Ownership of PD is released to the process (all threads may use it) + after the new thread starts in a joinable state + i.e. pthread_create returns a usable pthread_t. + + (2) Ownership of PD is released to the new thread starting in a detached + state. + + (3) Ownership of PD is dynamically released to a running thread via + pthread_detach. + + (4) Ownership of PD is acquired by the thread which calls pthread_join. + + Implementation notes: + + The PD->stopped_start and thread_ran variables are used to determine + exactly which of the four ownership states we are in and therefore + what actions can be taken. For example after (2) we cannot read or + write from PD anymore since the thread may no longer exist and the + memory may be unmapped. The most complicated cases happen during + thread startup: + + (a) If the created thread is in a detached (PTHREAD_CREATE_DETACHED), + or joinable (default PTHREAD_CREATE_JOINABLE) state and + STOPPED_START is true, then the creating thread has ownership of + PD until the PD->lock is released by pthread_create. If any + errors occur we are in states (c), (d), or (e) below. + + (b) If the created thread is in a detached state + (PTHREAD_CREATED_DETACHED), and STOPPED_START is false, then the + creating thread has ownership of PD until it invokes the OS + kernel's thread creation routine. If this routine returns + without error, then the created thread owns PD; otherwise, see + (c) and (e) below. + + (c) If the detached thread setup failed and THREAD_RAN is true, then + the creating thread releases ownership to the new thread by + sending a cancellation signal. All threads set THREAD_RAN to + true as quickly as possible after returning from the OS kernel's + thread creation routine. + + (d) If the joinable thread setup failed and THREAD_RAN is true, then + then the creating thread retains ownership of PD and must cleanup + state. Ownership cannot be released to the process via the + return of pthread_create since a non-zero result entails PD is + undefined and therefore cannot be joined to free the resources. + We privately call pthread_join on the thread to finish handling + the resource shutdown (Or at least we should, see bug 19511). + + (e) If the thread creation failed and THREAD_RAN is false, then the + creating thread retains ownership of PD and must cleanup state. + No waiting for the new thread is required because it never + started. + + The nptl_db interface: + + The interface with nptl_db requires that we enqueue PD into a linked + list and then call a function which the debugger will trap. The PD + will then be dequeued and control returned to the thread. The caller + at the time must have ownership of PD and such ownership remains + after control returns to thread. The enqueued PD is removed from the + linked list by the nptl_db callback td_thr_event_getmsg. The debugger + must ensure that the thread does not resume execution, otherwise + ownership of PD may be lost and examining PD will not be possible. + + Note that the GNU Debugger as of (December 10th 2015) commit + c2c2a31fdb228d41ce3db62b268efea04bd39c18 no longer uses + td_thr_event_getmsg and several other related nptl_db interfaces. The + principal reason for this is that nptl_db does not support non-stop + mode where other threads can run concurrently and modify runtime + structures currently in use by the debugger and the nptl_db + interface. + + Axioms: + + * The create_thread function can never set stopped_start to false. + * The created thread can read stopped_start but never write to it. + * The variable thread_ran is set some time after the OS thread + creation routine returns, how much time after the thread is created + is unspecified, but it should be as quickly as possible. + +*/ + +/* CREATE THREAD NOTES: + + createthread.c defines the create_thread function, and two macros: START_THREAD_DEFN and START_THREAD_SELF (see below). - create_thread is obliged to initialize PD->stopped_start. It - should be true if the STOPPED_START parameter is true, or if - create_thread needs the new thread to synchronize at startup for - some other implementation reason. If PD->stopped_start will be - true, then create_thread is obliged to perform the operation - "lll_lock (PD->lock, LLL_PRIVATE)" before starting the thread. + create_thread must initialize PD->stopped_start. It should be true + if the STOPPED_START parameter is true, or if create_thread needs the + new thread to synchronize at startup for some other implementation + reason. If STOPPED_START will be true, then create_thread is obliged + to lock PD->lock before starting the thread. Then pthread_create + unlocks PD->lock which synchronizes-with START_THREAD_DEFN in the + child thread which does an acquire/release of PD->lock as the last + action before calling the user entry point. The goal of all of this + is to ensure that the required initial thread attributes are applied + (by the creating thread) before the new thread runs user code. Note + that the the functions pthread_getschedparam, pthread_setschedparam, + pthread_setschedprio, __pthread_tpp_change_priority, and + __pthread_current_priority reuse the same lock, PD->lock, for a + similar purpose e.g. synchronizing the setting of similar thread + attributes. These functions are never called before the thread is + created, so don't participate in startup syncronization, but given + that the lock is present already and in the unlocked state, reusing + it saves space. The return value is zero for success or an errno code for failure. If the return value is ENOMEM, that will be translated to EAGAIN, so create_thread need not do that. On failure, *THREAD_RAN should be set to true iff the thread actually started up and then got - cancelled before calling user code (*PD->start_routine), in which - case it is responsible for doing its own cleanup. */ - + canceled before calling user code (*PD->start_routine). */ static int create_thread (struct pthread *pd, const struct pthread_attr *attr, - bool stopped_start, STACK_VARIABLES_PARMS, + bool *stopped_start, STACK_VARIABLES_PARMS, bool *thread_ran); #include <createthread.c> @@ -314,12 +430,19 @@ START_THREAD_DEFN /* Store the new cleanup handler info. */ THREAD_SETMEM (pd, cleanup_jmp_buf, &unwind_buf); + /* We are either in (a) or (b), and in either case we either own + PD already (2) or are about to own PD (1), and so our only + restriction would be that we can't free PD until we know we + have ownership (see CONCURRENCY NOTES above). */ if (__glibc_unlikely (pd->stopped_start)) { int oldtype = CANCEL_ASYNC (); /* Get the lock the parent locked to force synchronization. */ lll_lock (pd->lock, LLL_PRIVATE); + + /* We have ownership of PD now. */ + /* And give it up right away. */ lll_unlock (pd->lock, LLL_PRIVATE); @@ -378,7 +501,8 @@ START_THREAD_DEFN pd, pd->nextevent)); } - /* Now call the function to signal the event. */ + /* Now call the function which signals the event. See + CONCURRENCY NOTES for the nptl_db interface comments. */ __nptl_death_event (); } } @@ -642,19 +766,28 @@ __pthread_create_2_1 (pthread_t *newthread, const pthread_attr_t *attr, that cares whether the thread count is correct. */ atomic_increment (&__nptl_nthreads); - bool thread_ran = false; + /* Our local value of stopped_start and thread_ran can be accessed at + any time. The PD->stopped_start may only be accessed if we have + ownership of PD (see CONCURRENCY NOTES above). */ + bool stopped_start = false; bool thread_ran = false; /* Start the thread. */ if (__glibc_unlikely (report_thread_creation (pd))) { - /* Create the thread. We always create the thread stopped - so that it does not get far before we tell the debugger. */ - retval = create_thread (pd, iattr, true, STACK_VARIABLES_ARGS, - &thread_ran); + stopped_start = true; + + /* We always create the thread stopped at startup so we can + notify the debugger. */ + retval = create_thread (pd, iattr, &stopped_start, + STACK_VARIABLES_ARGS, &thread_ran); if (retval == 0) { - /* create_thread should have set this so that the logic below can - test it. */ + /* We retain ownership of PD until (a) (see CONCURRENCY NOTES + above). */ + + /* Assert stopped_start is true in both our local copy and the + PD copy. */ + assert (stopped_start); assert (pd->stopped_start); /* Now fill in the information about the new thread in @@ -671,26 +804,30 @@ __pthread_create_2_1 (pthread_t *newthread, const pthread_attr_t *attr, pd, pd->nextevent) != 0); - /* Now call the function which signals the event. */ + /* Now call the function which signals the event. See + CONCURRENCY NOTES for the nptl_db interface comments. */ __nptl_create_event (); } } else - retval = create_thread (pd, iattr, false, STACK_VARIABLES_ARGS, - &thread_ran); + retval = create_thread (pd, iattr, &stopped_start, + STACK_VARIABLES_ARGS, &thread_ran); if (__glibc_unlikely (retval != 0)) { - /* If thread creation "failed", that might mean that the thread got - created and ran a little--short of running user code--but then - create_thread cancelled it. In that case, the thread will do all - its own cleanup just like a normal thread exit after a successful - creation would do. */ - if (thread_ran) - assert (pd->stopped_start); + /* State (c) or (d) and we may not have PD ownership (see + CONCURRENCY NOTES above). We can assert that STOPPED_START + must have been true because thread creation didn't fail, but + thread attribute setting did. */ + /* See bug 19511 which explains why doing nothing here is a + resource leak for a joinable thread. */ + assert (stopped_start); else { + /* State (e) and we have ownership of PD (see CONCURRENCY + NOTES above). */ + /* Oops, we lied for a second. */ atomic_decrement (&__nptl_nthreads); @@ -710,10 +847,14 @@ __pthread_create_2_1 (pthread_t *newthread, const pthread_attr_t *attr, } else { - if (pd->stopped_start) - /* The thread blocked on this lock either because we're doing TD_CREATE - event reporting, or for some other reason that create_thread chose. - Now let it run free. */ + /* We don't know if we have PD ownership. Once we check the local + stopped_start we'll know if we're in state (a) or (b) (see + CONCURRENCY NOTES above). */ + if (stopped_start) + /* State (a), we own PD. The thread blocked on this lock either + because we're doing TD_CREATE event reporting, or for some + other reason that create_thread chose. Now let it run + free. */ lll_unlock (pd->lock, LLL_PRIVATE); /* We now have for sure more than one thread. The main thread might |