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    SubjectRe: [PATCH] Exiting queue and task might race to free cic
    Date
    Hi Jens

    On Thursday 20 November 2008 10:27:06 Nikanth Karthikesan wrote:
    > On Wednesday 19 November 2008 19:45:31 Jens Axboe wrote:
    > > On Wed, Nov 19 2008, Nikanth Karthikesan wrote:
    > >
    > > Not sure this is enough, we probably need to copy the key to ensure that
    > > we get a fresh value. How does this look?
    >
    > Agreed. Read barrier required. But the compiler hint, "likely" can stay?
    >
    > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
    > index 6a062ee..4504b94 100644
    > --- a/block/cfq-iosched.c
    > +++ b/block/cfq-iosched.c
    > @@ -1318,7 +1318,15 @@ 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 (likely(cic->key))
    > + __cfq_exit_single_io_context(cfqd, cic);
    > +
    > spin_unlock_irqrestore(q->queue_lock, flags);
    > }
    > }
    >
    > > Did you actually trigger this, or is it just from code inspection?
    >
    > No. But I am looking at another bug report on Suse Kernel where the bug is
    > triggered during reboot when the kernel thread usb_stor_scan_thread exits.
    >

    This patch seems to solve the above said bug report on Suse kernel. So, yes it
    is reproducible! Can this be merged?

    Thanks
    Nikanth



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