ACK/Cmnt: [SRU][Bionic][PATCH] blk-mq: silence false positive warnings in hctx_unlock()

[Andrea Righi]

On Fri, Jan 10, 2020 at 03:24:57PM -0800, Kamal Mostafa wrote:
> From: Jens Axboe <axboe at kernel.dk>
> 
> BugLink: https://bugs.launchpad.net/bugs/1848739
> 
> In some stupider versions of gcc, it complains:
> 
> 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:620:6: note: ‘srcu_idx’ was declared here
>   int srcu_idx;
>       ^
> 
> which is completely bogus, since we only use srcu_idx when
> hctx->flags & BLK_MQ_F_BLOCKING is set, and that's the case where
> hctx_lock() has initialized it.
> 
> Just set it to '0' in the normal path in hctx_lock() to silence
> this annoying warning.
> 
> Fixes: 04ced159cec8 ("blk-mq: move hctx lock/unlock into a helper")
> Fixes: 5197c05e16b4 ("blk-mq: protect completion path with RCU")
> Signed-off-by: Jens Axboe <axboe at kernel.dk>
> (backported from commit 08b5a6e2a769f720977b245431b45134c0bdd377)
> Signed-off-by: Kamal Mostafa <kamal at canonical.com>
> ---
> 
> BugLink: https://bugs.launchpad.net/bugs/1848739
> 
> Recent blk-mq patches to Bionic for this LP bug introduce more annoying
> instances of this false-positive build warning.  This trivial backport
> from mainline hushes them.
> 
> -Kamal

The bug report seems to be marked already as 'Fix Released' in bionic.
Maybe we need a new bug?

Apart than that:

Acked-by: Andrea Righi <andrea.righi at canonical.com>

> 
> ---
>  block/blk-mq.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 52d41d92555e..b4be8229a5cf 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -590,9 +590,11 @@ static void hctx_unlock(struct blk_mq_hw_ctx *hctx, int srcu_idx)
>  
>  static void hctx_lock(struct blk_mq_hw_ctx *hctx, int *srcu_idx)
>  {
> -	if (!(hctx->flags & BLK_MQ_F_BLOCKING))
> +	if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
> +		/* shut up gcc false positive */
> +		*srcu_idx = 0;
>  		rcu_read_lock();
> -	else
> +	} else
>  		*srcu_idx = srcu_read_lock(hctx->queue_rq_srcu);
>  }
>  
> -- 
> 2.17.1
> 
> 
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