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        Mon, 17 Oct 2016 15:40:27 -0400
From: Jeff Moyer <jmoyer@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: viro@zeniv.linux.org.uk, jack@suse.cz, dmonakhov@openvz.org,
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        Peter Zijlstra <peterz@infradead.org>,
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Subject: Re: [PATCH] aio: fix a use after free (and fix freeze protection of aio writes)
References: <1476597082-15317-1-git-send-email-hch@lst.de>
        <x49twcabzy4.fsf@segfault.boston.devel.redhat.com>
        <20161017185552.GA24653@lst.de>
Date: Mon, 17 Oct 2016 15:40:24 -0400
In-Reply-To: <20161017185552.GA24653@lst.de> (Christoph Hellwig's message of
        "Mon, 17 Oct 2016 20:55:52 +0200")
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Christoph Hellwig <hch@lst.de> writes:

> On Mon, Oct 17, 2016 at 02:19:47PM -0400, Jeff Moyer wrote:
>> This ends up being a call to __sb_end_write:
>> 
>> void __sb_end_write(struct super_block *sb, int level)
>> {
>>         percpu_up_read(sb->s_writers.rw_sem + level-1);
>> }
>> 
>> Nothing guarantees that submission and completion happen on the same
>> CPU.  Is this safe?
>
> Good point.  From my reading of the percpu_rwsem implementation it
> is not safe to release it from a different CPU.  Which makes me
> wonder how we can protect aio writes properly here..

Could we just change percpu_rw_semaphore->read_count to be a signed
integer?  The down_write path sums up the counters from all cpus...

-Jeff