path: root/src/thread/pthread_mutex_unlock.c
diff options
authorRich Felker <>2016-06-27 15:18:13 -0400
committerRich Felker <>2016-06-27 15:18:13 -0400
commit384d103d94dba0472a587861f67d7ed6e8955f86 (patch)
treeb113865f16b998f0c8d3d7fe19f33c39785a5e7b /src/thread/pthread_mutex_unlock.c
parent6cec7bc57f599f43f4041cec2093e3c9231dbaab (diff)
fix failure to obtain EOWNERDEAD status for process-shared robust mutexes
Linux's documentation (robust-futex-ABI.txt) claims that, when a process dies with a futex on the robust list, bit 30 (0x40000000) is set to indicate the status. however, what actually happens is that bits 0-30 are replaced with the value 0x40000000, i.e. bits 0-29 (containing the old owner tid) are cleared at the same time bit 30 is set. our userspace-side code for robust mutexes was written based on that documentation, assuming that kernel would never produce a futex value of 0x40000000, since the low (owner) bits would always be non-zero. commit d338b506e39b1e2c68366b12be90704c635602ce introduced this assumption explicitly while fixing another bug in how non-recoverable status for robust mutexes was tracked. presumably the tests conducted at that time only checked non-process-shared robust mutexes, which are handled in pthread_exit (which implemented the documented kernel protocol, not the actual one) rather than by the kernel. change pthread_exit robust list processing to match the kernel behavior, clearing bits 0-29 while setting bit 30, and use the value 0x7fffffff instead of 0x40000000 to encode non-recoverable status. the choice of value here is arbitrary; any value with at least one of bits 0-29 set should work just as well,
Diffstat (limited to 'src/thread/pthread_mutex_unlock.c')
1 files changed, 1 insertions, 1 deletions
diff --git a/src/thread/pthread_mutex_unlock.c b/src/thread/pthread_mutex_unlock.c
index 02da92a9..7dd00d27 100644
--- a/src/thread/pthread_mutex_unlock.c
+++ b/src/thread/pthread_mutex_unlock.c
@@ -24,7 +24,7 @@ int __pthread_mutex_unlock(pthread_mutex_t *m)
if (next != &self->robust_list.head) *(volatile void *volatile *)
((char *)next - sizeof(void *)) = prev;
- cont = a_swap(&m->_m_lock, (type & 8) ? 0x40000000 : 0);
+ cont = a_swap(&m->_m_lock, (type & 8) ? 0x7fffffff : 0);
if (type != PTHREAD_MUTEX_NORMAL && !priv) {
self->robust_list.pending = 0;