linux-kernel - Re: [PATCH] btrfs: Use refcount_t instead of atomic_t for mmap

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Message-ID: <021ca5c7-bdf8-4ec1-b59f-dce8333fe158@suse.com>
Date: Tue, 29 Apr 2025 17:12:27 +0930
From: Qu Wenruo <wqu@...e.com>
To: Bo Liu <liubo03@...pur.com>, clm@...com, josef@...icpanda.com,
 dsterba@...e.com
Cc: linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] btrfs: Use refcount_t instead of atomic_t for mmap_count



在 2025/4/29 16:50, Bo Liu 写道:
> Use an API that resembles more the actual use of mmap_count.
> Found by cocci:
> fs/btrfs/bio.c:153:5-24: WARNING: atomic_dec_and_test variation before object free at line 155

Please explain this better, I didn't see anything wrong about the 
decreasing the atomic to zero then freeing it.

Thanks,
Qu>
> Signed-off-by: Bo Liu <liubo03@...pur.com>
> ---
>   fs/btrfs/bio.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/btrfs/bio.c b/fs/btrfs/bio.c
> index a3ee9a976f6f..353c61936cd6 100644
> --- a/fs/btrfs/bio.c
> +++ b/fs/btrfs/bio.c
> @@ -23,7 +23,7 @@ static mempool_t btrfs_failed_bio_pool;
>   struct btrfs_failed_bio {
>   	struct btrfs_bio *bbio;
>   	int num_copies;
> -	atomic_t repair_count;
> +	refcount_t repair_count;
>   };
>   
>   /* Is this a data path I/O that needs storage layer checksum and repair? */
> @@ -150,7 +150,7 @@ static int prev_repair_mirror(struct btrfs_failed_bio *fbio, int cur_mirror)
>   
>   static void btrfs_repair_done(struct btrfs_failed_bio *fbio)
>   {
> -	if (atomic_dec_and_test(&fbio->repair_count)) {
> +	if (refcount_dec_and_test(&fbio->repair_count)) {
>   		btrfs_bio_end_io(fbio->bbio, fbio->bbio->bio.bi_status);
>   		mempool_free(fbio, &btrfs_failed_bio_pool);
>   	}
> @@ -235,10 +235,10 @@ static struct btrfs_failed_bio *repair_one_sector(struct btrfs_bio *failed_bbio,
>   		fbio = mempool_alloc(&btrfs_failed_bio_pool, GFP_NOFS);
>   		fbio->bbio = failed_bbio;
>   		fbio->num_copies = num_copies;
> -		atomic_set(&fbio->repair_count, 1);
> +		refcount_set(&fbio->repair_count, 1);
>   	}
>   
> -	atomic_inc(&fbio->repair_count);
> +	refcount_inc(&fbio->repair_count);
>   
>   	repair_bio = bio_alloc_bioset(NULL, 1, REQ_OP_READ, GFP_NOFS,
>   				      &btrfs_repair_bioset);