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SubjectRe: [PATCH] aio: fix a use after free (and fix freeze protection of aio writes)
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?

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