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Date: Mon, 17 Oct 2016 21:54:54 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Jeff Moyer <jmoyer@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>, viro@zeniv.linux.org.uk,
        jack@suse.cz, dmonakhov@openvz.org, linux-fsdevel@vger.kernel.org,
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        Oleg Nesterov <oleg@redhat.com>,
        Mikulas Patocka <mpatocka@redhat.com>
Subject: Re: [PATCH] aio: fix a use after free (and fix freeze protection of
 aio writes)
Message-ID: <20161017195454.GA3568@worktop.programming.kicks-ass.net>
References: <1476597082-15317-1-git-send-email-hch@lst.de>
 <x49twcabzy4.fsf@segfault.boston.devel.redhat.com>
 <20161017185552.GA24653@lst.de>
 <x49pomyydav.fsf@segfault.boston.devel.redhat.com>
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On Mon, Oct 17, 2016 at 03:40:24PM -0400, Jeff Moyer wrote:
> 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...

To what point?