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,

diff --git a/src/thread/pthread_create.c b/src/thread/pthread_create.c
index e7df34a..9f6b98e 100644 (file)
--- a/src/thread/pthread_create.c
+++ b/src/thread/pthread_create.c
@@ -79,7 +79,7 @@ _Noreturn void __pthread_exit(void *result)
                int priv = (m->_m_type & 128) ^ 128;
                self->robust_list.pending = rp;
                self->robust_list.head = *rp;
-               int cont = a_swap(&m->_m_lock, self->tid|0x40000000);
+               int cont = a_swap(&m->_m_lock, 0x40000000);
                self->robust_list.pending = 0;
                if (cont < 0 || waiters)
                        __wake(&m->_m_lock, 1, priv);

diff --git a/src/thread/pthread_mutex_trylock.c b/src/thread/pthread_mutex_trylock.c
index 0df3ce2..54876a6 100644 (file)
--- a/src/thread/pthread_mutex_trylock.c
+++ b/src/thread/pthread_mutex_trylock.c
@@ -14,7 +14,7 @@ int __pthread_mutex_trylock_owner(pthread_mutex_t *m)
                m->_m_count++;
                return 0;
        }
-       if (own == 0x40000000) return ENOTRECOVERABLE;
+       if (own == 0x7fffffff) return ENOTRECOVERABLE;
 
        if (m->_m_type & 128) {
                if (!self->robust_list.off) {

diff --git a/src/thread/pthread_mutex_unlock.c b/src/thread/pthread_mutex_unlock.c
index 02da92a..7dd00d2 100644 (file)
--- 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;
                __vm_unlock();