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.
--
Jens Axboe
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