linux-kernel - Re: [PATCH] Fix SUNRPC wakeup/execute race condition

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Date:	Sun, 05 Nov 2006 12:01:33 -0500
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Christophe Saout <christophe@...ut.de>
Cc:	linux-kernel@...r.kernel.org,
	NFS V4 Mailing List <nfsv4@...ux-nfs.org>,
	"J. Bruce Fields" <bfields@...i.umich.edu>
Subject: Re: [PATCH] Fix SUNRPC wakeup/execute race condition

On Sun, 2006-11-05 at 11:32 +0100, Christophe Saout wrote:
> Am Sonntag, den 05.11.2006, 01:50 -0500 schrieb Trond Myklebust:
> 
> > > --- linux-2.6.18/net/sunrpc/sched.c	2006-09-20 05:42:06.000000000 +0200
> > > +++ linux/net/sunrpc/sched.c	2006-11-04 20:38:56.000000000 +0100
> > > @@ -302,12 +302,9 @@ EXPORT_SYMBOL(__rpc_wait_for_completion_
> > >   */
> > >  static void rpc_make_runnable(struct rpc_task *task)
> > >  {
> > > -	int do_ret;
> > > -
> > >  	BUG_ON(task->tk_timeout_fn);
> > > -	do_ret = rpc_test_and_set_running(task);
> > >  	rpc_clear_queued(task);
> > > -	if (do_ret)
> > > +	if (rpc_test_and_set_running(task))
> > >  		return;
> > >  	if (RPC_IS_ASYNC(task)) {
> > >  		int status;
> > 
> > This fix looks wrong to me. If we've made it to 'rpc_make_runnable',
> > then the rpc_task will have already been removed from the
> > rpc_wait_queue.
> 
> I just flipped the two lines, changed nothing else. Why exactly do you
> think that's wrong, I don't see anything particular that could be broken
> by chaning the ordering. Anyway, the fsstress has been running for 18
> hours straight now without showing any signs of problems.

OK. I finally see the bug that you've spotted. The problem occurs when
__rpc_execute clears RPC_TASK_RUNNING after rpc_make_runnable has called
rpc_test_and_set_running, but before it has called rpc_clear_queued.

However if you just swap the two lines, you run into a new race:
__rpc_execute() may just put the rpc_task back to sleep before your call
to rpc_test_and_set_running() finishes executing.
We therefore need an extra test for RPC_IS_QUEUED() in
rpc_make_runnable().


Cheers,
  Trond

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