On 01/23/2017 07:07 AM, Hannes Reinecke wrote:
> Hi Jens,
>
> while hunting yet another queue stall I've been looking at
> __blk_mq_tag_busy():
>
> bool __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
> {
> if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) &&
> !test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
> atomic_inc(&hctx->tags->active_queues);
>
> return true;
> }
>
> Is this some subtle usage of bitops (and I just didn't grasp it) or is
> the first test_bit() pointless?
Is it subtle? I've used that in a few places in blk-mq, and in mm/filemap.c
as well:
commit 7fcbbaf18392f0b17c95e2f033c8ccf87eecde1d
Author: Jens Axboe <axboe@xxxxxx>
Date: Thu May 22 11:54:16 2014 -0700
mm/filemap.c: avoid always dirtying mapping->flags on O_DIRECT
the test_and_* functions always need to be locked, and there's no point
in doing that if we only rarely need to change the value.
> If the former I _really_ would like to have some comments on why this is
> required.
Sure, but I really didn't think it was black magic.
--
Jens Axboe
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