linux-kernel - Re: [PATCH 0/3] aio: ctx->dead cleanups

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Message-ID: <20150617003906.GC17109@ZenIV.linux.org.uk>
Date:	Wed, 17 Jun 2015 01:39:06 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Jeff Moyer <jmoyer@...hat.com>, linux-aio@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] aio: ctx->dead cleanups

On Wed, Jun 17, 2015 at 01:04:14AM +0200, Oleg Nesterov wrote:
> Al, please help. We are trying to backport some aio fixes and I am
> absolutely confused by your b2edffdd912b "fix mremap() vs. ioctx_kill()
> race".
> 
> 
> Firstly, I simply can't understand what exactly it tries to fix. OK,
> aio_free_ring() can race with kill and we can remap the soon-to-be-killed
> ctx. So what? kill_ioctx() will the the correct (already re-mapped)
> ctx->mmap_base after it drops mm->ioctx_lock.

Huh?  kill_ioctx() picks ctx->mmap_base and passes it to vm_munmap().
Which tries to grab mmap_sem, blocks for mremap() from another thread
and waits for it to drop mmap_sem.  By that time ctx->mmap_base has
nothing whatsoever to the argument we'd passed to vm_munmap().  Sure,
it had been recalculated by aio_ring_remap(), but it's too late for
us - we'd already fetched the old value.
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