linux-kernel - Re: [PATCH 2/8] blk-mq: protect completion path with RCU

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Date:   Mon, 8 Jan 2018 15:55:38 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Holger Hoffstätte <holger@...lied-asynchrony.com>,
        Tejun Heo <tj@...nel.org>, jack@...e.cz, clm@...com,
        jbacik@...com
Cc:     kernel-team@...com, linux-kernel@...r.kernel.org,
        linux-btrfs@...r.kernel.org, peterz@...radead.org,
        jianchao.w.wang@...cle.com, Bart.VanAssche@....com,
        linux-block@...r.kernel.org
Subject: Re: [PATCH 2/8] blk-mq: protect completion path with RCU

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 <tj@...nel.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