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 <[email protected]>
---
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);
}
}
Or this would also work
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 6a062ee..7a068bd 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1318,7 +1318,11 @@ 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);
+ /*
+ * Make sure the queue is not dead.
+ */
+ if (likely(!test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
+ __cfq_exit_single_io_context(cfqd, cic);
spin_unlock_irqrestore(q->queue_lock, flags);
}
}
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 <[email protected]>
>
> ---
> 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
> From: Jens Axboe <[email protected]>
> Date: Wed, Nov 19, 2008 03:15:31PM +0100
>
> 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 <[email protected]>
> >
> > ---
> > 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);
> }
> }
>
I've seen once the oops reported (the BUG() now @ line 1247), but I've
never been able to reproduce it afterwards. I think that there still
is a window open for a race here:
1314 struct cfq_data *cfqd = cic->key;
1315
=====> here cfq_exit_queue() can free cfqd and assign cic->key = NULL,
and accessing cfqd->queue is not safe. [ If I'm not wrong :) ]
1316 if (cfqd) {
1317 struct request_queue *q = cfqd->queue;
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.
Thanks
Nikanth Karthikesan
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