On Jul 22, 2015, at 5:14 PM, Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx> wrote:
> On Wed, Jul 22, 2015 at 4:40 PM, Chuck Lever <chuck.lever@xxxxxxxxxx> wrote:
>> Hi Trond-
>>
>>
>> On Jul 22, 2015, at 4:36 PM, Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx> wrote:
>>
>>> xprt_alloc_bc_request() cannot call xprt_free_bc_request() without
>>> deadlocking, since it already holds the xprt->bc_pa_lock.
>>>
>>> Reported-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
>>> Fixes: 0d2a970d0ae55 ("SUNRPC: Fix a backchannel race")
>>> Signed-off-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
>>
>> That’s exactly what I did as a basic fix, and I can report that
>> it successfully avoids the deadlock.
>>
>> If xprt_alloc_bc_request() no longer calls xprt_free_bc_request(),
>> are the accounting changes introduced by 0d2a970d0ae55 still
>> necessary?
>
> The accounting changes are there to fix the race that Christoph
> reported in which a valid backchannel request can be rejected by the
> RPC layer if it happens to come in after the reply to the previous
> request was sent, but before xprt_free_bc_request has completed. I
> would therefore expect them still to be needed.
>
> That said, I just noticed that the bc_free_count was being incorrectly
> maintained. Should be fixed with v2 of the patch series.
I will get v2 in front of the tester who found this issue.
--
Chuck Lever
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