linux-kernel - Re: [External] Re: [PATCH] mm/z3fold: remove unneeded spinlock

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Date: Thu, 8 Feb 2024 04:29:08 +0100
From: Johannes Weiner <hannes@...xchg.org>
To: Zhongkun He <hezhongkun.hzk@...edance.com>
Cc: Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Vitaly Wool <vitaly.wool@...sulko.com>
Subject: Re: [External] Re: [PATCH] mm/z3fold: remove unneeded spinlock

On Mon, Feb 05, 2024 at 09:08:05AM +0800, Zhongkun He wrote:
> On Mon, Feb 5, 2024 at 2:46 AM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > On Sun, Feb 04, 2024 at 08:54:04PM +0800, Zhongkun He wrote:
> > > There is no need to use spinlock in this section, so
> > > remove it.
> >
> > I don't know this code at all, but the idiom is (relatively) common.
> > It waits until anybody _currently_ holding the lock has released it.
> >
> > That would, eg, make it safe to free the 'pool' memory.
> >
> > > -     spin_lock(&pool->lock);
> > > -     spin_unlock(&pool->lock);
> >
> 
> no,  please see the commit 'e774a7bc7f0adb'.
> 
>         spin_lock(&pool->lock);
> -       if (!list_empty(&page->lru))
> -               list_del_init(&page->lru);
>         spin_unlock(&pool->lock);
> 
> The original purpose of this lock was to protect  page->lru,
> which was removed now, so the spinlock is unnecessary.

But pool->lock protects other stuff too? This doesn't rule out that
there is some other ordering dependency on cycling the lock before
freeing the entry. The person who would know best is the maintainer of
this code, Vitaly. Let's CC him.