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_pthread_cancel() breaks the normal lock order of first locking the

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joined and then the joiner thread. There isn't an easy (sane?) way
to make it use the correct order without introducing races involving
the target thread and finding which (active or dead) list it is on. So,
after locking the canceled thread it will try to lock the joined thread
and if it fails release the first lock and try again from the top.

Introduce a new function, _spintrylock, which is simply a wrapper arround
umtx_trylock(), to help accomplish this.

Approved by: re/blanket libthr
This commit is contained in:
mtm 2003-05-25 08:48:11 +00:00
parent e182f494ed
commit 4c97496368
3 changed files with 28 additions and 4 deletions
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21
lib/libthr/thread/thr_cancel.c
View File

@ -21,7 +21,16 @@ int
_pthread_cancel(pthread_t pthread)
{
int ret;
pthread_t joined;
/*
* When canceling a thread that has joined another thread, this
* routine breaks the normal lock order of locking first the
* joined and then the joiner. Therefore, it is necessary that
* if it can't obtain the second lock, that it release the first
* one and restart from the top.
*/
retry:
if ((ret = _find_thread(pthread)) != 0)
/* The thread is not on the list of active threads */
goto out;
@ -70,10 +79,14 @@ _pthread_cancel(pthread_t pthread)
/*
* Disconnect the thread from the joinee:
*/
if (pthread->join_status.thread != NULL) {
pthread->join_status.thread->joiner
= NULL;
pthread->join_status.thread = NULL;
if ((joined = pthread->join_status.thread) != NULL) {
if (_spintrylock(&joined->lock) == EBUSY) {
_thread_critical_exit(pthread);
goto retry;
}
pthread->join_status.thread->joiner = NULL;
_spinunlock(&joined->lock);
joined = pthread->join_status.thread = NULL;
}
pthread->cancelflags |= PTHREAD_CANCELLING;
PTHREAD_NEW_STATE(pthread, PS_RUNNING);

1
lib/libthr/thread/thr_private.h
View File

@ -737,6 +737,7 @@ int _pthread_mutexattr_settype(pthread_mutexattr_t *, int);
int _pthread_once(pthread_once_t *, void (*) (void));
pthread_t _pthread_self(void);
int _pthread_setspecific(pthread_key_t, const void *);
int _spintrylock(spinlock_t *);
inline void _spinlock_pthread(pthread_t, spinlock_t *);
inline void _spinunlock_pthread(pthread_t, spinlock_t *);
void _thread_exit(char *, int, char *);

10
lib/libthr/thread/thr_spinlock.c
View File

@ -69,6 +69,16 @@ _spinlock(spinlock_t *lck)
_spinlock_pthread(curthread, lck);
}
int
_spintrylock(spinlock_t *lck)
{
int error;
error = umtx_trylock((struct umtx *)lck, curthread->thr_id);
if (error != 0 && error != EBUSY)
abort();
return (error);
}
inline void
_spinlock_pthread(pthread_t pthread, spinlock_t *lck)
{