Messages in this thread | | | Date | Sat, 01 Jan 2005 17:48:09 +0100 | From | Guilhem Lavaux <> | Subject | PThreads, signals, futex and SMP |
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Hi,
I am one of the developer of kaffe, a GPL implementation of the Java Virtual Machine, and we encountered some problems on Linux/2.6/SMP. I am currently running 2.6.8.1 Mandrake Kernel. I can try whether the problem is reproduceable on a vanilla kernel but I don't think this part has been touched.
Here is the problem: we have a regression test which is quite intense in thread creation/destruction, each thread can start the garbage collector. The garbage collector needs to stop all running thread to be able to walk the heap/stack. For this, it uses a particular signal which is sent to all threads. The signal handler calls sigwait to stop the thread. 50% of the time everything is fine but from time to time, kaffe has a deadlock. It appears that it always happen when we are in the following configuration:
Thread 1 ------------
sigwait <signal handler> futex syscall pthread_mutex_unlock
Thread 2 ------------ futex syscall pthread_mutex_lock Garbage Collector thread
Now if we look at the futex code in the linux kernel, we see this:
static int futex_wait(unsigned long uaddr, int val, unsigned long time) { DECLARE_WAITQUEUE(wait, current); int ret, curval; struct futex_q q;
down_read(¤t->mm->mmap_sem);
the kernel then prepares the wait queue and unlock mmap_sem.
Concerning the mutex_unlock part we have this:
static int futex_wake(unsigned long uaddr, int nr_wake) { union futex_key key; struct futex_hash_bucket *bh; struct list_head *head; struct futex_q *this, *next; int ret;
down_read(¤t->mm->mmap_sem);
and the kernel iterates the semaphores and wakes up all threads.
What may happen if the signal handler is called after down_read in futex_wake ? My guess is that we are not able to call futex_wait because the application will deadlock because the first thread is frozen by a sigwait.
So either we have a limitation of the kernel either a bug if the analysis is correct. The only point is that I am not sure whether a signal is allowed to interrupt a syscall just in the middle of futex_wake. If this is not possible there may be a bug in our application somewhere else.
Regards,
Guilhem Lavaux. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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