Re: [Xen-devel] [PATCH, v2] xenstore: Fix deadlock in xs_read_watch

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 30 Aug 2010, Daniel De Graaf wrote:
> On 08/30/2010 09:16 AM, Stefano Stabellini wrote:
> > On Thu, 26 Aug 2010, Daniel De Graaf wrote:
> >> When read_message returns -1 while a read is pending, an attempt is made
> >> to lock h->request_mutex which is already locked by the reader. Change
> >> the read thread to only exit on thread cancellation, where
> >> read_thr_exists will return 0.
> >>
> >> Signed-off-by: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
> >>
> >> diff -r 6bebaf40e925 tools/xenstore/xs.c
> >> --- a/tools/xenstore/xs.c  Tue Jul 20 13:42:17 2010 +0100
> >> +++ b/tools/xenstore/xs.c  Thu Aug 26 17:08:55 2010 -0400
> >> @@ -271,6 +271,7 @@
> >>  {
> >>  #ifdef USE_PTHREAD
> >>    if (h->read_thr_exists) {
> >> +          h->read_thr_exists = 0;
> >>            pthread_cancel(h->read_thr);
> >>            pthread_join(h->read_thr, NULL);
> >>    }
> >> @@ -667,7 +668,7 @@
> >>    mutex_lock(&h->watch_mutex);
> >>  
> >>    /* Wait on the condition variable for a watch to fire. */
> >> -  while (list_empty(&h->watch_list) && read_thread_exists(h))
> >> +  while (list_empty(&h->watch_list) && (!read_from_thread || 
> >> read_thread_exists(h)))
> >>            condvar_wait(&h->watch_condvar, &h->watch_mutex, h);
> >>    if (!read_thread_exists(h)) {
> >>            mutex_unlock(&h->watch_mutex);
> > 
> > read_from_thread is not declared in this function
> > 
> 
> Sorry about that, looks like I picked the wrong patch to send. Corrected
> patch attached.
> 

According to the description you gave, this patch fixes a problem
occurring when read_message returns -1 while a read is pending in
read_thread that is relevant only when USE_PTHREAD.
Why are you changing a code path that is only compiled when
!USE_PTHREAD?
Also shouldn't you be checking the return value of read_message anyway?
Considering that h->read_thr_exists is set to zero only on
xs_daemon_close, and in that case pthread_cancel is called right after,
I doubt that any of the operation run in read_thread after the loop will
be executed.


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