linux-kernel - Re: [PATCH] btrfs: zstd ensure reclaim timer is properly cleaned up

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Message-ID: <20190222165357.GA85619@dennisz-mbp>
Date:   Fri, 22 Feb 2019 11:53:57 -0500
From:   Dennis Zhou <dennis@...nel.org>
To:     Nikolay Borisov <nborisov@...e.com>
Cc:     Dennis Zhou <dennis@...nel.org>, David Sterba <dsterba@...e.com>,
        Josef Bacik <josef@...icpanda.com>, Chris Mason <clm@...com>,
        Omar Sandoval <osandov@...ndov.com>,
        Nick Terrell <terrelln@...com>, kernel-team@...com,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] btrfs: zstd ensure reclaim timer is properly cleaned up

On Fri, Feb 22, 2019 at 02:17:41PM +0200, Nikolay Borisov wrote:
> 
> 
> 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.
> 

Hmmm, I see. I misunderstood the invariant. I'll spin v2 and handle it
another way.

Thanks,
Dennis

> > 
> > Signed-off-by: Dennis Zhou <dennis@...nel.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])) {
> >