lkml.org 
[lkml]   [2008]   [May]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: 2.6.26-rc4: RIP __call_for_each_cic+0x20/0x50
On Thu, May 29 2008, Paul E. McKenney wrote:
> On Thu, May 29, 2008 at 01:44:31PM +0200, Jens Axboe wrote:
> > On Thu, May 29 2008, Paul E. McKenney wrote:
> > > On Thu, May 29, 2008 at 12:13:54PM +0200, Jens Axboe wrote:
> > > > On Thu, May 29 2008, Paul E. McKenney wrote:
> > > > > On Thu, May 29, 2008 at 08:42:02AM +0200, Jens Axboe wrote:
> > > > > > On Thu, May 29 2008, Jens Axboe wrote:
> > > > > > > > But one additional question...
> > > > > > > >
> > > > > > > > static void cfq_cic_free_rcu(struct rcu_head *head)
> > > > > > > > {
> > > > > > > > struct cfq_io_context *cic;
> > > > > > > >
> > > > > > > > cic = container_of(head, struct cfq_io_context, rcu_head);
> > > > > > > >
> > > > > > > > kmem_cache_free(cfq_ioc_pool, cic);
> > > > > > > > elv_ioc_count_dec(ioc_count);
> > > > > > > >
> > > > > > > > if (ioc_gone && !elv_ioc_count_read(ioc_count))
> > > > > > > > complete(ioc_gone);
> > > > > > > > }
> > > > > > > >
> > > > > > > > Suppose that a pair of tasks both execute the elv_ioc_count_dec()
> > > > > > > > at the same time, so that all counters are now zero. Both then
> > > > > > > > find that there is still an ioc_gone, and that the count is
> > > > > > > > now zero. One of the tasks invokes complete(ioc_gone). This
> > > > > > > > awakens the corresponding cfq_exit(), which now returns, getting
> > > > > > > > rid of its stack frame -- and corrupting the all_gone auto variable
> > > > > > > > that ioc_gone references.
> > > > > > > >
> > > > > > > > Now the second task gets a big surprise when it tries to invoke
> > > > > > > > complete(ioc_gone).
> > > > > > > >
> > > > > > > > Or is there something else that I am missing here?
> > > > > > >
> > > > > > > No, I think that's a problem spot as well. To my knowledge, nobody has
> > > > > > > ever hit that. The anticipatory scheduler has the same code.
> > > > > > >
> > > > > > > What we want to avoid here is making cfq_cic_free_rcu() a lot more
> > > > > > > expensive, which is why the elv_ioc_count_read() is behind that
> > > > > > > ioc_gone check. I'll need to think a bit on how to handle that
> > > > > > > better :-)
> > > > > >
> > > > > > So how about this? Add a spinlock for checking and clearing ioc_gone
> > > > > > back to NULL. It doesn't matter if we make the ioc_gone != NULL
> > > > > > case a little more expensive, as it will only happen on cfq-iosched
> > > > > > module unload. And it seems the clearest way of making this safe.
> > > > > > The last hunk should really not be necessary, as ioc_gone wont be
> > > > > > set back to NULL before wait_for_completion() is entered.
> > > > >
> > > > > Looks better! I do have one scenario that seems troublesome, but
> > > > > it should be easy to fix, see below. (Assuming it really is a
> > > > > problem, that is...)
> > > > >
> > > > > Thanx, Paul
> > > > >
> > > > > > An identical patch is needed in AS as well.
> > > > > >
> > > > > > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> > > > > > index d01b411..32aa367 100644
> > > > > > --- a/block/cfq-iosched.c
> > > > > > +++ b/block/cfq-iosched.c
> > > > > > @@ -48,6 +48,7 @@ static struct kmem_cache *cfq_ioc_pool;
> > > > > >
> > > > > > static DEFINE_PER_CPU(unsigned long, ioc_count);
> > > > > > static struct completion *ioc_gone;
> > > > > > +static DEFINE_SPINLOCK(ioc_gone_lock);
> > > > > >
> > > > > > #define CFQ_PRIO_LISTS IOPRIO_BE_NR
> > > > > > #define cfq_class_idle(cfqq) ((cfqq)->ioprio_class == IOPRIO_CLASS_IDLE)
> > > > > > @@ -1177,8 +1178,19 @@ static void cfq_cic_free_rcu(struct rcu_head *head)
> > > > > > kmem_cache_free(cfq_ioc_pool, cic);
> > > > > > elv_ioc_count_dec(ioc_count);
> > > > > >
> > > > > > - if (ioc_gone && !elv_ioc_count_read(ioc_count))
> > > > > > - complete(ioc_gone);
> > > > > > + if (ioc_gone) {
> > > > > > + /*
> > > > > > + * CFQ scheduler is exiting, grab exit lock and check
> > > > > > + * the pending io context count. If it hits zero,
> > > > > > + * complete ioc_gone and set it back to NULL
> > > > > > + */
> > > > >
> > > > > Suppose that at this point some other CPU does the last complete().
> > > > > They have set ioc_gone to NULL, so everything is fine. But suppose
> > > > > that in the meantime, some other CPU sets up a cfq and then starts
> > > > > tearing it down. Then ioc_gone would be non-NULL, and we would cause
> > > > > this new teardown to end prematurely.
> > > > >
> > > > > If this is a real problem, one way to get around it is to have a
> > > > > generation number. We capture this before doing the elv_ioc_count_dec()
> > > > > (alas, with a memory barrier between the capture and the elv_ioc_count_dec()),
> > > > > and then check it under the lock. If it has changed, we know someone else
> > > > > has already done the awakening for us. Increment the generation number
> > > > > in the same place that ioc_gone is set to NULL.
> > > > >
> > > > > Seem reasonable?
> > > >
> > > > This isn't a problem, since cfq_exit() cannot be called before
> > > > all block queues in the system have been detached from CFQ.
> > >
> > > And once all block queues have been detached, no future block queues
> > > can ever be attached again? Or perhaps a better way of putting it,
> > > once CFQ has been shut down, can it be restarted without rebooting
> > > the system? If it can be restarted without reboot, then I do not
> > > yet see how the scenario above is avoided.
> >
> > Certainly, you can modprobe cfq-iosched and switch any device to cfq
> > again. But I really don't see the problem in this case. Once CFQ
> > has exited, all tasks are detached from CFQ. They have to be, or
> > we could not unload the module.
>
> Ah, OK -- so we cannot reload the module until the earlier
> instantiation has been unloaded, which cannot happen until everyone
> has returned from any code in the CFQ module. If it is linked
> directly into the kernel, it can only be "unloaded" by bringing the
> system down.

Right!

> Sorry for my confusion!
>
> This approach looks good to me.

Pheew good, I was starting to have a brain melt down as well :-)

--
Jens Axboe



\
 
 \ /
  Last update: 2008-05-29 14:17    [W:0.060 / U:1.928 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site