On Sun, 2006-11-05 at 18:42 +0100, Christophe Saout wrote:
> On Sun, 2006-11-05 at 12:01 -0500, Trond Myklebust wrote:
> > 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.
>
> Yes, exactly.
>
> > 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().
>
> Damn, you're right. I missed that one. What about that:
>
> ----
> The sunrpc scheduler contains a race condition that can let an RPC
> task end up being neither running nor on any wait queue. The race takes
> place between rpc_make_runnable (called from rpc_wake_up_task) and
> __rpc_execute under the following condition:
>
> First __rpc_execute calls tk_action which puts the task on some wait
> queue. The task is dequeued by another process before __rpc_execute
> continues its execution. While executing rpc_make_runnable exactly after
> setting the task `running' bit and before clearing the `queued' bit
> __rpc_execute picks up execution, clears `running' and subsequently
> both functions fall through, both under the false assumption somebody
> else took the job.
>
> Swapping rpc_test_and_set_running with rpc_clear_queued in
> rpc_make_runnable fixes that hole. This introduces another possible
> race condition that can be handled by checking for `queued' after
> setting the `running' bit.
>
> Bug noticed on a 4-way x86_64 system under XEN with an NFSv4 server
> on the same physical machine, apparently one of the few ways to hit
> this race condition at all.
>
> Cc: Trond Myklebust <[email protected]>
> Cc: J. Bruce Fields <[email protected]>
> Signed-off-by: Christophe Saout <[email protected]>
>
> --- 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 +302,15 @@ 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;
> + /* We might have raced */
> + if (RPC_IS_QUEUED(task)) {
> + rpc_clear_running(task);
> + return;
> + }
> if (RPC_IS_ASYNC(task)) {
> int status;
This looks OK.
> - 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;
> + /* We might have raced with __rpc_execute */
> + if (RPC_IS_QUEUED(task)) {
> + rpc_clear_running(task);
> + return;
> + }
> if (RPC_IS_ASYNC(task)) {
> int status;
A cut'n paste error?
Cheers,
Trond
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
