linux-kernel - Re: [PATCH] btrfs: zstd: use spin

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Date:   Mon, 11 Apr 2022 15:51:37 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Schspa Shi <schspa@...il.com>
Cc:     clm@...com, Josef Bacik <josef@...icpanda.com>, dsterba@...e.com,
        terrelln@...com, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] btrfs: zstd: use spin_lock in timer function

On Sat, Apr 09, 2022 at 03:36:54PM +0800, Schspa Shi wrote:
> David Sterba <dsterba@...e.cz> writes:
> 
> > On Sat, Apr 09, 2022 at 02:15:23AM +0800, Schspa Shi wrote:
> >> timer callback was running on bh, and there is no need to disable bh again.
> >
> > Why do you think so? There was a specific fix fee13fe96529 ("btrfs:
> > correct zstd workspace manager lock to use spin_lock_bh()") that
> > actually added the _bh, so either you need to explain why exactly it's
> > not needed anymore and verify that the reported lockdep warning from the
> > fix does not happen.
> 
> Yes, I've seen this fix, and wsm.lru_list is protected by wsm.lock.
> This patch will not remove all changes that were fixed. Just a little
> improvement
> to remove the unnecessary bh disabling. Like
> static inline void red_adaptative_timer(struct timer_list *t)
> in net/sched/sch_red.c.
> 
> Because the critical section is only used by the process context and
> the softirq context,
> it is safe to remove bh_disable in the softirq context since it will
> not be preempted by the softirq.

So why haven't you written that as a proper explanation the first time,
you apparenly analyzed the correctness. Please update the changelog and
also please try to rephrase it so it's more readable, I kind of
understand what you mean but it still leaves some things to hard to
read. Thanks.