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Subject: Re: [PATCH] btrfs: zstd ensure reclaim timer is properly cleaned up
To:     Dennis Zhou <dennis@kernel.org>, David Sterba <dsterba@suse.com>,
        Josef Bacik <josef@toxicpanda.com>, Chris Mason <clm@fb.com>,
        Omar Sandoval <osandov@osandov.com>,
        Nick Terrell <terrelln@fb.com>
Cc:     kernel-team@fb.com, linux-btrfs@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20190221202556.42447-1-dennis@kernel.org>
From:   Nikolay Borisov <nborisov@suse.com>
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Message-ID: <eb7df5c4-25fb-2d10-58a1-b3be7b37c2db@suse.com>
Date:   Fri, 22 Feb 2019 14:17:41 +0200
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On 21.02.19 г. 22:25 ч., Dennis Zhou wrote:
> The timer function, zstd_reclaim_timer_fn(), reschedules itself under
> certain conditions. Switch to del_timer_sync() to ensure that the timer
> function hasn't rescheduled itself.

According to del_timer_sync it just waits for any concurrent invocation
to finish. But it's responsibility of the caller to ensure the timer
cannot really be restarted. It's not obvious how
zstd_cleanup_workspace_manager ensures that the timer won't be restared.
Looking at the timer function for it to not restart wsm.lru_list has to
be empty. And seeing that workspace->lru_list is deleted afterwards I'm
not sure this invariant holds.

> 
> Signed-off-by: Dennis Zhou <dennis@kernel.org>
> ---
>  fs/btrfs/zstd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c
> index 3e418a3aeb11..62de9a211321 100644
> --- a/fs/btrfs/zstd.c
> +++ b/fs/btrfs/zstd.c
> @@ -195,7 +195,7 @@ static void zstd_cleanup_workspace_manager(void)
>  	struct workspace *workspace;
>  	int i;
>  
> -	del_timer(&wsm.timer);
> +	del_timer_sync(&wsm.timer);
>  
>  	for (i = 0; i < ZSTD_BTRFS_MAX_LEVEL; i++) {
>  		while (!list_empty(&wsm.idle_ws[i])) {
>