Cygwin: convert malloc lock to SRWLOCK

Per https://cygwin.com/pipermail/cygwin-developers/2021-October/012429.html,
we may encounter a crash when starting multiple threads during process
startup (here: fhandler_fifo::fixup_after_{fork,exec}) which in turn
allocate memory via malloc.

The problem is concurrent usage of malloc before the malloc muto has
been initialized.

To fix this issue, convert the muto to a SRWLOCK and make sure it is
statically initalized.  Thus, malloc can be called as early as necessary
and malloc_init is only required to check for user space provided malloc.

Note that this requires to implement a __malloc_trylock macro to be
called from fork.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

7 files changed, 12 insertions, 9 deletions

diff --git a/winsup/cygwin/cygmalloc.h b/winsup/cygwin/cygmalloc.h
index 84bad82..36f5629 100644
--- a/winsup/cygwin/cygmalloc.h
+++ b/winsup/cygwin/cygmalloc.h
@@ -36,9 +36,11 @@ void *mmap64 (void *, size_t, int, int, int, off_t);
 
 #elif defined (__INSIDE_CYGWIN__)
 
-# define __malloc_lock() mallock.acquire ()
-# define __malloc_unlock() mallock.release ()
-extern muto mallock;
+# define __malloc_lock() AcquireSRWLockExclusive (&mallock)
+# define __malloc_trylock() TryAcquireSRWLockExclusive (&mallock)
+# define __malloc_unlock() ReleaseSRWLockExclusive (&mallock)
+extern SRWLOCK NO_COPY mallock;
+void malloc_init ();
 
 #endif
 
diff --git a/winsup/cygwin/dcrt0.cc b/winsup/cygwin/dcrt0.cc
index 6f4723b..f3d09c1 100644
--- a/winsup/cygwin/dcrt0.cc
+++ b/winsup/cygwin/dcrt0.cc
@@ -29,6 +29,7 @@ details. */
 #include "shared_info.h"
 #include "cygwin_version.h"
 #include "dll_init.h"
+#include "cygmalloc.h"
 #include "heap.h"
 #include "tls_pbuf.h"
 #include "exception.h"
diff --git a/winsup/cygwin/fork.cc b/winsup/cygwin/fork.cc
index 7192178..912f172 100644
--- a/winsup/cygwin/fork.cc
+++ b/winsup/cygwin/fork.cc
@@ -296,7 +296,7 @@ frok::parent (volatile char * volatile stack_here)
   si.lpReserved2 = (LPBYTE) &ch;
   si.cbReserved2 = sizeof (ch);
 
-  bool locked = __malloc_lock ();
+  bool locked = __malloc_trylock ();
 
   /* Remove impersonation */
   cygheap->user.deimpersonate ();
diff --git a/winsup/cygwin/heap.cc b/winsup/cygwin/heap.cc
index b839c8c..f27f81b 100644
--- a/winsup/cygwin/heap.cc
+++ b/winsup/cygwin/heap.cc
@@ -230,7 +230,6 @@ user_heap_info::init ()
   debug_printf ("heap base %p, heap top %p, heap size %ly (%lu)",
 		base, top, chunk, chunk);
   page_const--;
-  // malloc_init ();
 }
 
 #define pround(n) (((size_t)(n) + page_const) & ~page_const)
diff --git a/winsup/cygwin/heap.h b/winsup/cygwin/heap.h
index 565758e..25200f2 100644
--- a/winsup/cygwin/heap.h
+++ b/winsup/cygwin/heap.h
@@ -10,7 +10,6 @@ details. */
 
 /* Heap management. */
 void heap_init ();
-void malloc_init ();
 
 #define inheap(s) \
   (cygheap->user_heap.ptr && s \
diff --git a/winsup/cygwin/malloc_wrapper.cc b/winsup/cygwin/malloc_wrapper.cc
index 3b24580..2842e53 100644
--- a/winsup/cygwin/malloc_wrapper.cc
+++ b/winsup/cygwin/malloc_wrapper.cc
@@ -269,13 +269,11 @@ strdup (const char *s)
    newlib will call __malloc_lock and __malloc_unlock at appropriate
    times.  */
 
-muto NO_COPY mallock;
+SRWLOCK NO_COPY mallock = SRWLOCK_INIT;
 
 void
 malloc_init ()
 {
-  mallock.init ("mallock");
-
   /* Check if malloc is provided by application. If so, redirect all
      calls to malloc/free/realloc to application provided. This may
      happen if some other dll calls cygwin's malloc, but main code provides
diff --git a/winsup/cygwin/release/3.3.0 b/winsup/cygwin/release/3.3.0
index 895c273..1595b17 100644
--- a/winsup/cygwin/release/3.3.0
+++ b/winsup/cygwin/release/3.3.0
@@ -78,3 +78,7 @@ Bug Fixes
 - Fix access violation that can sometimes occur when copy/pasting between
   32-bit and 64-bit Cygwin environments.  Align clipboard descriptor layouts.
   Addresses: https://cygwin.com/pipermail/cygwin-patches/2021q4/011517.html
+
+- Fix a synchronization issue when running multiple threads from DLL
+  initialization which in turn call malloc.
+  Addresses: https://cygwin.com/pipermail/cygwin/2021-October/249635.html