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@xxxxxxxxxx> >>> --- >>> 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