On 03/28/2014 07:41 PM, Sage Weil wrote:
> Hi Alex, Ilya,
>
> I've added this and the previous patch to a for-linus branch to send to
> Linux for 3.14. The net of the two patches is simply removing the assert,
> however... the first changes several lines that then get changed back.
> Should we squash them?
In my opinion, yes. Ilya's movement of the assert within
the spinlock was solving one problem, but ultimately that
assertion should go away.
-Alex
> Thanks!
> sage
>
>
> On Wed, 26 Mar 2014, Alex Elder wrote:
>
>> Olivier Bonvalet reported having repeated crashes due to a failed
>> assertion he was hitting in rbd_img_obj_callback():
>>
>> Assertion failure in rbd_img_obj_callback() at line 2165:
>> rbd_assert(which == img_request->next_completion);
>>
>> With a lot of help from Olivier with reproducing the problem
>> we were able to determine the object and image requests had
>> already been completed (and often freed) at the point the
>> assertion failed.
>>
>> There was a great deal of discussion on the ceph-devel mailing list
>> about this. The problem only arose when there were two (or more)
>> object requests in an image request, and the problem was always
>> seen when the second request was being completed.
>>
>> The problem is due to a race in the window between setting the
>> "done" flag on an object request and checking the image request's
>> next completion value. When the first object request completes, it
>> checks to see if its successor request is marked "done", and if
>> so, that request is also completed. In the process, the image
>> request's next_completion value is updated to reflect that both
>> the first and second requests are completed. By the time the
>> second request is able to check the next_completion value, it
>> has been set to a value *greater* than its own "which" value,
>> which caused an assertion to fail.
>>
>> Fix this problem by skipping over any completion processing
>> unless the completing object request is the next one expected.
>> Test only for inequality (not >=), and eliminate the bad
>> assertion.
>>
>> Tested-by: Olivier Bonvalet <[email protected]>
>> Signed-off-by: Alex Elder <[email protected]>
>> ---
>> drivers/block/rbd.c | 3 +--
>> 1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>> index f044fab..4c95b50 100644
>> --- a/drivers/block/rbd.c
>> +++ b/drivers/block/rbd.c
>> @@ -2125,10 +2125,9 @@ static void rbd_img_obj_callback(struct
>> rbd_obj_request *obj_request)
>> rbd_assert(which < img_request->obj_request_count);
>>
>> spin_lock_irq(&img_request->completion_lock);
>> - if (which > img_request->next_completion)
>> + if (which != img_request->next_completion)
>> goto out;
>>
>> - rbd_assert(which == img_request->next_completion);
>> for_each_obj_request_from(img_request, obj_request) {
>> rbd_assert(more);
>> rbd_assert(which < img_request->obj_request_count);
>> --
>> 1.7.9.5
>>
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