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Apparently there are some popular programs around which assume that it

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is safe to call pthread_mutex_init() on the same shared mutex several
times.  POSIX claims that the behaviour in this case is undefined.

Make this working by only allowing one caller to initialize the mutex.
Other callers either see already completed initialization and do
nothing, or busy-loop yielding while designated initializer finishes.
Also make the API requirements loose by initializing mutexes on other
pthread_mutex*() calls if they see uninitialized shared mutex.

Only mutexes provide the hack for now, but it could be also
implemented for other process shared primitives from libthr.

Reported and tested by:	"Oleg V. Nauman" <oleg@opentransfer.com>
Sponsored by:	The FreeBSD Foundation
This commit is contained in:
Konstantin Belousov 2016-03-22 10:51:42 +00:00
parent f6c7371c81
commit 6044c03a37
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=297185
2 changed files with 61 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

55
lib/libthr/thread/thr_mutex.c
View File

@ -38,6 +38,7 @@
* $FreeBSD$
*/
#include <stdbool.h>
#include "namespace.h"
#include <stdlib.h>
#include <errno.h>
@ -264,6 +265,51 @@ set_inherited_priority(struct pthread *curthread, struct pthread_mutex *m)
m->m_lock.m_ceilings[1] = -1;
}
static void
shared_mutex_init(struct pthread_mutex *pmtx, const struct
pthread_mutex_attr *mutex_attr)
{
static const struct pthread_mutex_attr foobar_mutex_attr = {
.m_type = PTHREAD_MUTEX_DEFAULT,
.m_protocol = PTHREAD_PRIO_NONE,
.m_ceiling = 0,
.m_pshared = PTHREAD_PROCESS_SHARED
};
bool done;
/*
* Hack to allow multiple pthread_mutex_init() calls on the
* same process-shared mutex. We rely on kernel allocating
* zeroed offpage for the mutex, i.e. the
* PMUTEX_INITSTAGE_ALLOC value must be zero.
*/
for (done = false; !done;) {
switch (pmtx->m_ps) {
case PMUTEX_INITSTAGE_DONE:
atomic_thread_fence_acq();
done = true;
break;
case PMUTEX_INITSTAGE_ALLOC:
if (atomic_cmpset_int(&pmtx->m_ps,
PMUTEX_INITSTAGE_ALLOC, PMUTEX_INITSTAGE_BUSY)) {
if (mutex_attr == NULL)
mutex_attr = &foobar_mutex_attr;
mutex_init_body(pmtx, mutex_attr);
atomic_store_rel_int(&pmtx->m_ps,
PMUTEX_INITSTAGE_DONE);
done = true;
}
break;
case PMUTEX_INITSTAGE_BUSY:
_pthread_yield();
break;
default:
PANIC("corrupted offpage");
break;
}
}
}
int
__pthread_mutex_init(pthread_mutex_t *mutex,
const pthread_mutexattr_t *mutex_attr)
@ -285,7 +331,7 @@ __pthread_mutex_init(pthread_mutex_t *mutex,
if (pmtx == NULL)
return (EFAULT);
*mutex = THR_PSHARED_PTR;
mutex_init_body(pmtx, *mutex_attr);
shared_mutex_init(pmtx, *mutex_attr);
return (0);
}
@ -426,6 +472,7 @@ check_and_init_mutex(pthread_mutex_t *mutex, struct pthread_mutex **m)
*m = __thr_pshared_offpage(mutex, 0);
if (*m == NULL)
ret = EINVAL;
shared_mutex_init(*m, NULL);
} else if (__predict_false(*m <= THR_MUTEX_DESTROYED)) {
if (*m == THR_MUTEX_DESTROYED) {
ret = EINVAL;
@ -588,6 +635,7 @@ _pthread_mutex_unlock(pthread_mutex_t *mutex)
mp = __thr_pshared_offpage(mutex, 0);
if (mp == NULL)
return (EINVAL);
shared_mutex_init(mp, NULL);
} else {
mp = *mutex;
}
@ -815,6 +863,7 @@ _pthread_mutex_getprioceiling(pthread_mutex_t *mutex,
m = __thr_pshared_offpage(mutex, 0);
if (m == NULL)
return (EINVAL);
shared_mutex_init(m, NULL);
} else {
m = *mutex;
if (m <= THR_MUTEX_DESTROYED)
@ -839,6 +888,7 @@ _pthread_mutex_setprioceiling(pthread_mutex_t *mutex,
m = __thr_pshared_offpage(mutex, 0);
if (m == NULL)
return (EINVAL);
shared_mutex_init(m, NULL);
} else {
m = *mutex;
if (m <= THR_MUTEX_DESTROYED)
@ -942,12 +992,13 @@ __pthread_mutex_setyieldloops_np(pthread_mutex_t *mutex, int count)
int
_pthread_mutex_isowned_np(pthread_mutex_t *mutex)
{
struct pthread_mutex *m;
struct pthread_mutex *m;
if (*mutex == THR_PSHARED_PTR) {
m = __thr_pshared_offpage(mutex, 0);
if (m == NULL)
return (0);
shared_mutex_init(m, NULL);
} else {
m = *mutex;
if (m <= THR_MUTEX_DESTROYED)

8
lib/libthr/thread/thr_private.h
View File

@ -146,6 +146,13 @@ TAILQ_HEAD(mutex_queue, pthread_mutex);
#define MAX_DEFER_WAITERS 50
/*
* Values for pthread_mutex m_ps indicator.
*/
#define PMUTEX_INITSTAGE_ALLOC 0
#define PMUTEX_INITSTAGE_BUSY 1
#define PMUTEX_INITSTAGE_DONE 2
struct pthread_mutex {
/*
* Lock for accesses to this structure.
@ -156,6 +163,7 @@ struct pthread_mutex {
int m_count;
int m_spinloops;
int m_yieldloops;
int m_ps; /* pshared init stage */
/*
* Link for all mutexes a thread currently owns, of the same
* prio type.