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Date: Wed, 20 Dec 2006 23:58:43 -0800 From: "Chen, Kenneth W" <kenneth.w.chen@...el.com> To: "'Andrew Morton'" <akpm@...l.org>, <linux-aio@...ck.org> Cc: "'Trond Myklebust'" <trond.myklebust@....uio.no>, "'xb'" <xavier.bru@...l.net>, "'Zach Brown'" <zach.brown@...cle.com>, <linux-kernel@...r.kernel.org>, "Paul E. McKenney" <paulmck@...ibm.com> Subject: RE: [patch] aio: fix buggy put_ioctx call in aio_complete 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@...el.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@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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