Messages in this thread Patch in this message | | | Date | Wed, 19 Nov 2008 15:15:31 +0100 | From | Jens Axboe <> | Subject | Re: [PATCH] Exiting queue and task might race to free cic |
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On Wed, Nov 19 2008, Nikanth Karthikesan wrote: > Hi Jens > > Looking at the bug reported here > http://thread.gmane.org/gmane.linux.kernel/722539 > it looks like an exiting queue can race with an exiting task. > > When a queue exits the queue lock is taken and cfq_exit_queue() would free all > the cic's associated with the queue. > > But when a task exits, cfq_exit_io_context() gets cic one by one and then > locks the associated queue to call __cfq_exit_single_io_context. It looks like > between getting a cic from the ioc and locking the queue, the queue might have > exited on another cpu. Isn't this possible? > > If possible, either verifying whether cic->key is still not null or q->flags > does not have QUEUE_FLAG_DEAD set would fix this. > > Thanks > Nikanth Karthikesan > > Signed-off-by: Nikanth Karthikesan <knikanth@suse.de> > > --- > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c > index 6a062ee..b9b627a 100644 > --- a/block/cfq-iosched.c > +++ b/block/cfq-iosched.c > @@ -1318,7 +1318,12 @@ static void cfq_exit_single_io_context(struct > io_context *ioc, > unsigned long flags; > > spin_lock_irqsave(q->queue_lock, flags); > - __cfq_exit_single_io_context(cfqd, cic); > + /* > + * cic might have been already exited when an exiting task > + * races with an exiting queue. > + */ > + if (likely(cic->key)) > + __cfq_exit_single_io_context(cfqd, cic); > spin_unlock_irqrestore(q->queue_lock, flags); > } > }
Not sure this is enough, we probably need to copy the key to ensure that we get a fresh value. How does this look?
Did you actually trigger this, or is it just from code inspection?
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index 6a062ee..560cd1c 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -1318,7 +1318,14 @@ static void cfq_exit_single_io_context(struct io_context *ioc, unsigned long flags; spin_lock_irqsave(q->queue_lock, flags); - __cfq_exit_single_io_context(cfqd, cic); + + /* + * Ensure we get a fresh copy of the ->key to prevent + * race between exiting task and queue + */ + smp_read_barrier_depends(); + if (cic->key) + __cfq_exit_single_io_context(cfqd, cic); spin_unlock_irqrestore(q->queue_lock, flags); } } -- Jens Axboe
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