linux-kernel - Re: [PATCH] aio: ensure access to ctx->ring

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Date:	Mon, 24 Mar 2014 18:56:54 +0800
From:	Gu Zheng <guz.fnst@...fujitsu.com>
To:	Benjamin LaHaise <bcrl@...ck.org>
CC:	Tang Chen <tangchen@...fujitsu.com>, Dave Jones <davej@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>, jmoyer@...hat.com,
	kosaki.motohiro@...fujitsu.com,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	miaox@...fujitsu.com, linux-aio@...ck.org,
	fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] aio: ensure access to ctx->ring_pages is correctly serialised

Hi Ben,
On 03/22/2014 02:35 AM, Benjamin LaHaise wrote:

> Hi all,
> 
> Based on the issues reported by Tang and Gu, I've come up with the an 
> alternative fix that avoids adding additional locking in the event read 
> code path.  The fix is to take the ring_lock mutex during page migration, 
> which is already used to syncronize event readers and thus does not add 
> any new locking requirements in aio_read_events_ring().  I've dropped 
> the patches from Tang and Gu as a result.  This patch is now in my 
> git://git.kvack.org/~bcrl/aio-next.git tree and will be sent to Linus 
> once a few other people chime in with their reviews of this change.  
> Please review Tang, Gu.  Thanks!

As I mentioned before:
https://lkml.org/lkml/2014/3/20/34
We can put put_aio_ring_file() at the first of the
context teardown flow (aio_free_ring). Then, page migration and ctx freeing
will have mutual execution guarded by lock_page() v.s. truncate().
So that, the additional spinlock(address_space's private_lock) used to
protect use and updates of the mapping's private_data, and the sane check
of context is needless, so does the additional percpu_ref_get/put(&ctx->users).
But the enlarge ring_lock protection region to remove the additional spin_lock
is an elegant method if we ignore the effect to reading events while migrating
page is going.

Thanks,
Gu

> 
> 		-ben

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