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Add pthread mutex deadlock detection for debug builds #24607

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Add pthread mutex deadlock detection for debug builds #24607

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3 changes: 3 additions & 0 deletions system/lib/libc/musl/src/thread/pthread_mutex_lock.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2,9 +2,12 @@

int __pthread_mutex_lock(pthread_mutex_t *m)
{
#if !defined(__EMSCRIPTEN__) || defined(NDEBUG)
/* XXX EMSCRIPTEN always take the slow path in debug builds so we can trap rather than deadlock */
if ((m->_m_type&15) == PTHREAD_MUTEX_NORMAL
&& !a_cas(&m->_m_lock, 0, EBUSY))
return 0;
#endif
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Should upstream do the same for all platforms? I'm just trying to understand if this is a general improvement or if it works around a specific problem in wasm.

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I think its a general improvement for debug builds yes. I guess most libc's don't ship debug builds like we do (especially automatically selecting debug libs based on command line flags, which emcc does is quite unusual).

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Upstream musl doesn't really have any/many assertions AFAICT, so its not something they tend to consider I suppose?

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Indeed, it looks like upstream musl only has assertion in 2 very specific places:

src/regex/regcomp.c
src/malloc/mallocng/..

No other assertions in whole of libc.

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I see, thanks. Makes sense.


return __pthread_mutex_timedlock(m, 0);
}
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10 changes: 10 additions & 0 deletions system/lib/libc/musl/src/thread/pthread_mutex_timedlock.c
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Original file line number Diff line number Diff line change
@@ -1,5 +1,9 @@
#include "pthread_impl.h"

#ifdef __EMSCRIPTEN__
#include <assert.h>
#endif

#ifndef __EMSCRIPTEN__
#define IS32BIT(x) !((x)+0x80000000ULL>>32)
#define CLAMP(x) (int)(IS32BIT(x) ? (x) : 0x7fffffffU+((0ULL+(x))>>63))
Expand Down Expand Up @@ -57,9 +61,12 @@ static int pthread_mutex_timedlock_pi(pthread_mutex_t *restrict m, const struct

int __pthread_mutex_timedlock(pthread_mutex_t *restrict m, const struct timespec *restrict at)
{
#if !defined(__EMSCRIPTEN__) || defined(NDEBUG)
/* XXX EMSCRIPTEN always take the slow path in debug builds so we can trap rather than deadlock */
if ((m->_m_type&15) == PTHREAD_MUTEX_NORMAL
&& !a_cas(&m->_m_lock, 0, EBUSY))
return 0;
#endif

int type = m->_m_type;
int r, t, priv = (type & 128) ^ 128;
Expand All @@ -79,6 +86,9 @@ int __pthread_mutex_timedlock(pthread_mutex_t *restrict m, const struct timespec
int own = r & 0x3fffffff;
if (!own && (!r || (type&4)))
continue;
#if defined(__EMSCRIPTEN__) && !defined(NDEBUG)
assert(own != __pthread_self()->tid && "pthread mutex deadlock detected");
#endif
if ((type&3) == PTHREAD_MUTEX_ERRORCHECK
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Can we perhaps enable this PTHREAD_MUTEX_ERRORCHECK globally somehow, rather than add these three lines? (It wouldn't assert though, just return EDEADLK)

&& own == __pthread_self()->tid)
return EDEADLK;
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3 changes: 3 additions & 0 deletions system/lib/libc/musl/src/thread/pthread_mutex_trylock.c
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Expand Up @@ -71,8 +71,11 @@ int __pthread_mutex_trylock_owner(pthread_mutex_t *m)

int __pthread_mutex_trylock(pthread_mutex_t *m)
{
#if !defined(__EMSCRIPTEN__) || defined(NDEBUG)
/* XXX EMSCRIPTEN always take the slow path in debug builds so we can trap rather than deadlock */
if ((m->_m_type&15) == PTHREAD_MUTEX_NORMAL)
return a_cas(&m->_m_lock, 0, EBUSY) & EBUSY;
#endif
return __pthread_mutex_trylock_owner(m);
}

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9 changes: 5 additions & 4 deletions system/lib/pthread/library_pthread.c
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Expand Up @@ -138,13 +138,9 @@ weak_alias(dummy_tsd, __pthread_tsd_main);
// See system/lib/README.md for static constructor ordering.
__attribute__((constructor(48)))
void _emscripten_init_main_thread(void) {
_emscripten_init_main_thread_js(&__main_pthread);

// The pthread struct has a field that points to itself - this is used as
// a magic ID to detect whether the pthread_t structure is 'alive'.
__main_pthread.self = &__main_pthread;
__main_pthread.stack = (void*)emscripten_stack_get_base();
__main_pthread.stack_size = emscripten_stack_get_base() - emscripten_stack_get_end();
__main_pthread.detach_state = DT_JOINABLE;
// pthread struct robust_list head should point to itself.
__main_pthread.robust_list.head = &__main_pthread.robust_list.head;
Expand All @@ -157,6 +153,11 @@ void _emscripten_init_main_thread(void) {
__main_pthread.next = __main_pthread.prev = &__main_pthread;
__main_pthread.tsd = (void **)__pthread_tsd_main;

_emscripten_init_main_thread_js(&__main_pthread);

__main_pthread.stack = (void*)emscripten_stack_get_base();
__main_pthread.stack_size = emscripten_stack_get_base() - emscripten_stack_get_end();
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Shouldn't the stack setup be as early as possible? Why move it down?


_emscripten_thread_mailbox_init(&__main_pthread);
_emscripten_thread_mailbox_await(&__main_pthread);
}
20 changes: 20 additions & 0 deletions test/other/test_pthread_mutex_deadlock.c
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@@ -0,0 +1,20 @@
#include <assert.h>
#include <pthread.h>
#include <stdbool.h>
#include <stdio.h>

pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;

int main() {
printf("in main\n");
int rtn = pthread_mutex_lock(&m);
assert(rtn == 0);

// Attempt to lock a second time. In debug builds this should
// hit an assertion. In release builds this will deadlock and
// never return.
pthread_mutex_lock(&m);
printf("should never get here\n");
assert(false);
return 0;
}
6 changes: 6 additions & 0 deletions test/test_other.py
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Expand Up @@ -12946,6 +12946,12 @@ def test_pthread_reuse(self):
def test_pthread_hello(self, args):
self.do_other_test('test_pthread_hello.c', args)

@crossplatform
@node_pthreads
def test_pthread_mutex_deadlock(self):
self.do_runf('other/test_pthread_mutex_deadlock.c', 'pthread mutex deadlock detected',
cflags=['-g'], assert_returncode=NON_ZERO)

@node_pthreads
def test_pthread_relocatable(self):
self.do_run_in_out_file_test('hello_world.c', cflags=['-sRELOCATABLE'])
Expand Down