linux-kernel - Re: [PATCH v2] btrfs: zstd ensure reclaim timer is properly cleaned up

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Message-ID: <20190227182916.GA18373@dennisz-mbp.dhcp.thefacebook.com>
Date:   Wed, 27 Feb 2019 13:29:16 -0500
From:   Dennis Zhou <dennis@...nel.org>
To:     David Sterba <dsterba@...e.cz>
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>,
        Nikolay Borisov <nborisov@...e.com>, kernel-team@...com,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] btrfs: zstd ensure reclaim timer is properly cleaned
 up

On Wed, Feb 27, 2019 at 05:44:41PM +0100, David Sterba wrote:
> On Fri, Feb 22, 2019 at 02:53:48PM -0500, Dennis Zhou wrote:
> > The timer function, zstd_reclaim_timer_fn(), reschedules itself under
> > certain conditions. When cleaning up, take the lock and remove all
> > workspaces. This prevents the timer from rearming itself. Lastly, switch
> > to del_timer_sync() to ensure that the timer function can't trigger as
> > we're unloading.
> > 
> > Signed-off-by: Dennis Zhou <dennis@...nel.org>
> 
> Reviewed-by: David Sterba <dsterba@...e.com>
> 

Thanks!

> > ---
> > v2:
> > - cleanup workspaces and then disable the timer
> > 
> >  fs/btrfs/zstd.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c
> > index 3e418a3aeb11..6b9e29d050f3 100644
> > --- a/fs/btrfs/zstd.c
> > +++ b/fs/btrfs/zstd.c
> > @@ -195,8 +195,7 @@ static void zstd_cleanup_workspace_manager(void)
> >  	struct workspace *workspace;
> >  	int i;
> >  
> > -	del_timer(&wsm.timer);
> > -
> > +	spin_lock(&wsm.lock);
> >  	for (i = 0; i < ZSTD_BTRFS_MAX_LEVEL; i++) {
> >  		while (!list_empty(&wsm.idle_ws[i])) {
> >  			workspace = container_of(wsm.idle_ws[i].next,
> > @@ -206,6 +205,9 @@ static void zstd_cleanup_workspace_manager(void)
> >  			wsm.ops->free_workspace(&workspace->list);
> 
> I've noticed while reading the code, why do you use the indirect call
> here? The wsm.ops points to btrfs_zstd_compress so free_workspace is
> always zstd_free_workspace.
> 
> The compiler is usually smart to replace such things by direct call if
> the type has not escaped, but this is not true for btrfs_compress_op so
> the indirect function call must be preserved.

I don't have a strong reason to use the indirect call here. It was just
to make it consistent for everyone to use the indirection. This at least
is in the cleanup path, so I don't think performance is that important?
But I don't feel strongly for or against calling zstd_free_workspace()
directly.

Thanks,
Dennis