On 1/8/18 1:15 PM, Jens Axboe wrote:
> On 1/8/18 12:57 PM, Holger Hoffstätte wrote:
>> On 01/08/18 20:15, Tejun Heo wrote:
>>> Currently, blk-mq protects only the issue path with RCU. This patch
>>> puts the completion path under the same RCU protection. This will be
>>> used to synchronize issue/completion against timeout by later patches,
>>> which will also add the comments.
>>>
>>> Signed-off-by: Tejun Heo <t...@kernel.org>
>>> ---
>>> block/blk-mq.c | 5 +++++
>>> 1 file changed, 5 insertions(+)
>>>
>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>> index ddc9261..6741c3e 100644
>>> --- a/block/blk-mq.c
>>> +++ b/block/blk-mq.c
>>> @@ -584,11 +584,16 @@ static void hctx_lock(struct blk_mq_hw_ctx *hctx, int
>>> *srcu_idx)
>>> void blk_mq_complete_request(struct request *rq)
>>> {
>>> struct request_queue *q = rq->q;
>>> + struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, rq->mq_ctx->cpu);
>>> + int srcu_idx;
>>>
>>> if (unlikely(blk_should_fake_timeout(q)))
>>> return;
>>> +
>>> + hctx_lock(hctx, &srcu_idx);
>>> if (!blk_mark_rq_complete(rq))
>>> __blk_mq_complete_request(rq);
>>> + hctx_unlock(hctx, srcu_idx);
>>> }
>>> EXPORT_SYMBOL(blk_mq_complete_request);
>>
>> So I've had v3 running fine with 4.14++ and when I first tried Jens'
>> additional helpers on top, I got a bunch of warnings which I didn't
>> investigate further at the time. Now they are back since the helpers
>> moved into patch #1 and #2 correctly says:
>>
>> ..
>> block/blk-mq.c: In function ‘blk_mq_complete_request’:
>> ./include/linux/srcu.h:175:2: warning: ‘srcu_idx’ may be used uninitialized
>> in this function [-Wmaybe-uninitialized]
>> __srcu_read_unlock(sp, idx);
>> ^~~~~~~~~~~~~~~~~~~~~~~~~~~
>> block/blk-mq.c:587:6: note: ‘srcu_idx’ was declared here
>> int srcu_idx;
>> ^~~~~~~~
>> ..etc.
>>
>> This is with gcc 7.2.0.
>>
>> I understand that this is a somewhat-false positive since the lock always
>> precedes the unlock & writes to the value, but can we properly initialize
>> or annotate this?
>
> It's not a somewhat false positive, it's a false positive. I haven't seen
> that bogus warning with the compiler I'm running:
>
> gcc (Ubuntu 7.2.0-1ubuntu1~16.04) 7.2.0
>
> and
>
> gcc (GCC) 7.2.0
>
> Neither of them throw the warning.
Are you on non-x86? Really bothers me to have to add a work-around
for something that's obviously a false positive.
I forget if we have some gcc/compiler annotation for this, otherwise
the good old
int srcu_idx = srcu_idx;
should get the job done.
--
Jens Axboe
