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SubjectRe: bio refcount problem
On Fri, Feb 18 2005, Philip R Auld wrote:
> Hi,
> I think there are some potential issues with the reference
> counting of bios as used in 2.6.10. The __make_request function
> which is the default block device routine accesses the bio structure
> after issuing the call to add_request. This means that the bio could
> have completed before __make_request uses it.
>
> The submit_bh path takes an extra reference with an explicit
> bio_get/put pair around the submit_bio, but many other users of
> submit_bio do not. Given that most of the end_io routines remove a
> reference and hence could free the bio this can lead at the least to
> __make_request mis-reading the sync flag. In more extreme cases it can
> cause an oops when run with CONFIG_DEBUG_PAGEALLOC.
>
> The question is what is the preferred fix? I think it may be to simply
> have submit_bio take its own reference (and remove the extra one from
> submit_bh).
>
> Alternatively __make_request could be adjusted so that it does not
> access the bio after calling add_request. All it is doing is checking
> the bi_rw field for the sync bit.
>
> Or make all users of submit_bio take and release and extra reference
> like submit_bh.

The queue lock is still held at that point, so the driver hasn't had a
chance to process the request yet.

--
Jens Axboe

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