Subject: Re: [PATCH 2/8] blk-mq: protect completion path with RCU
To: =?UTF-8?Q?Holger_Hoffst=c3=a4tte?= <holger@applied-asynchrony.com>,
        Tejun Heo <tj@kernel.org>, jack@suse.cz, clm@fb.com,
        jbacik@fb.com
Cc: kernel-team@fb.com, linux-kernel@vger.kernel.org,
        linux-btrfs@vger.kernel.org, peterz@infradead.org,
        jianchao.w.wang@oracle.com, Bart.VanAssche@wdc.com,
        linux-block@vger.kernel.org
References: <20180108191542.379478-1-tj@kernel.org>
 <20180108191542.379478-3-tj@kernel.org>
 <774b9fa0-c1f9-f528-09c3-2f3bd67e0fd5@applied-asynchrony.com>
From: Jens Axboe <axboe@kernel.dk>
Message-ID: <93157cfd-7c3c-c260-14e2-25784fe43314@kernel.dk>
Date: Mon, 8 Jan 2018 13:15:40 -0700
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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@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