PThreads, signals, futex and SMP
From: Guilhem Lavaux
Date: Sat Jan 01 2005 - 11:49:36 EST
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.
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