linux-kernel - Re: [PATCH v2] btrfs: zstd: use spin

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Date:   Wed, 13 Apr 2022 18:00:47 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Nikolay Borisov <nborisov@...e.com>
Cc:     Schspa Shi <schspa@...il.com>, clm@...com, dsterba@...e.com,
        josef@...icpanda.com, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, terrelln@...com
Subject: Re: [PATCH v2] btrfs: zstd: use spin_lock in timer callback

On Wed, Apr 13, 2022 at 05:58:41PM +0300, Nikolay Borisov wrote:
> 
> 
> On 11.04.22 г. 18:55 ч., Schspa Shi wrote:
> > This is an optimization for fix fee13fe96529 ("btrfs:
> > correct zstd workspace manager lock to use spin_lock_bh()")
> > 
> > The critical region for wsm.lock is only accessed by the process context and
> > the softirq context.
> > 
> > Because in the soft interrupt, the critical section will not be preempted by the
> > soft interrupt again, there is no need to call spin_lock_bh(&wsm.lock) to turn
> > off the soft interrupt, spin_lock(&wsm.lock) is enough for this situation.
> > 
> > Changelog:
> > v1 -> v2:
> > 	- Change the commit message to make it more readable.
> > 
> > [1] https://lore.kernel.org/all/20220408181523.92322-1-schspa@gmail.com/
> > 
> > Signed-off-by: Schspa Shi <schspa@...il.com>
> 
> Has there been any measurable impact by this change? While it's correct it does mean that
>   someone looking at the code would see that in one call site we use plain spinlock and in
> another a _bh version and this is somewhat inconsistent.

I think it would be hard to measure the impact, maybe in some kind of
load the _bh version would be unnecessarily blocking some other threads.

Regarding the used locking primitives, I'll add a comment about that to
the function, it is indeed inconsistent and not obvious from the
context.