lkml.org 
[lkml]   [2006]   [Dec]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: [patch] aio: fix buggy put_ioctx call in aio_complete
Date
Andrew Morton wrote on Wednesday, December 20, 2006 8:06 PM
> On Tue, 19 Dec 2006 13:49:18 -0800
> "Chen, Kenneth W" <kenneth.w.chen@intel.com> wrote:
> > Regarding to a bug report on:
> > http://marc.theaimsgroup.com/?l=linux-kernel&m=116599593200888&w=2
> >
> > flush_workqueue() is not allowed to be called in the softirq context.
> > However, aio_complete() called from I/O interrupt can potentially call
> > put_ioctx with last ref count on ioctx and trigger a bug warning. It
> > is simply incorrect to perform ioctx freeing from aio_complete.
> >
> > This patch removes all duplicate ref counting for each kiocb as
> > reqs_active already used as a request ref count for each active ioctx.
> > This also ensures that buggy call to flush_workqueue() in softirq
> > context is eliminated. wait_for_all_aios currently will wait on last
> > active kiocb. However, it is racy. This patch also tighten it up
> > by utilizing rcu synchronization mechanism to ensure no further
> > reference to ioctx before put_ioctx function is run.
>
> hrm, maybe. Does this count as "abuse of the RCU interfaces". Or "reuse"?

Yeah, it's abuse.

Problem is in wait_for_all_aios(), it is checking wait status without
properly holding an ioctx lock. Perhaps, this patch is walking on thin
ice. It abuses rcu over a buggy code. OTOH, I really don't want to hold
ctx_lock over the entire wakeup call at the end of aio_complete:

if (waitqueue_active(&ctx->wait))
wake_up(&ctx->wait);

I'm worried about longer lock hold time in aio_complete and potentially
increase lock contention for concurrent I/O completion. A quick look
at lockmeter data I had on a 4 socket system (with dual core + HT), it
already showing signs of substantial lock contention in aio_complete.
I'm afraid putting the above call inside ioctx_lock will make things
worse.

And synchronize_rcu fits the bill perfectly: aio_complete sets wakeup
status, drop ioctx_lock, do the wakeup call all protected inside rcu
lock. Then wait_for_all_aios will just wait for all that sequence to
complete before it proceed with __put_ioctx(). All nice and easy.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-12-21 09:01    [W:0.053 / U:0.564 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site