On Wed, Aug 31, 2016 at 03:03:24PM -0700, Bart Van Assche wrote:
> Protect sw-ctx busy bit manipulations via the sw queue lock.
> This allows to convert the atomic bit operations into slightly
> faster non-atomic operations. Document that blk_mq_run_hw_queues()
> tests these bits without holding the sw queue lock.
>
> Signed-off-by: Bart Van Assche <bart.vanassche@xxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Sagi Grimberg <sagi@xxxxxxxxxxx>
> ---
> block/blk-mq.c | 16 +++++++++++++---
> 1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 13f5a6c..0dcafa6 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -66,8 +66,9 @@ static void blk_mq_hctx_mark_pending(struct blk_mq_hw_ctx *hctx,
> {
> struct blk_align_bitmap *bm = get_bm(hctx, ctx);
>
> + lockdep_assert_held(&ctx->lock);
> if (!test_bit(CTX_TO_BIT(hctx, ctx), &bm->word))
> - set_bit(CTX_TO_BIT(hctx, ctx), &bm->word);
> + __set_bit(CTX_TO_BIT(hctx, ctx), &bm->word);
> }
>
> static void blk_mq_hctx_clear_pending(struct blk_mq_hw_ctx *hctx,
> @@ -75,7 +76,8 @@ static void blk_mq_hctx_clear_pending(struct blk_mq_hw_ctx *hctx,
> {
> struct blk_align_bitmap *bm = get_bm(hctx, ctx);
>
> - clear_bit(CTX_TO_BIT(hctx, ctx), &bm->word);
> + lockdep_assert_held(&ctx->lock);
> + __clear_bit(CTX_TO_BIT(hctx, ctx), &bm->word);
> }
NAK, we can't protect each bit with a separate lock, multiple software
queues share the bitmap. There's a race if we do non-atomic bit
operations on two software queues with bits in the same word.
--
Omar
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