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

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