linux-kernel - Re: [PATCH v3] cgroup: llist: avoid memory tears for llist

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Message-ID: <6isu4w3ri66t7kli43hpw4mehjvhipzndktbxrar3ttccap6jy@q2f2xdimxpga>
Date: Mon, 7 Jul 2025 11:23:49 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Tejun Heo <tj@...nel.org>, "Paul E . McKenney" <paulmck@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, JP Kobryn <inwardvessel@...il.com>, 
	Johannes Weiner <hannes@...xchg.org>, Ying Huang <huang.ying.caritas@...il.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Alexei Starovoitov <ast@...nel.org>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, bpf@...r.kernel.org, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH v3] cgroup: llist: avoid memory tears for llist_node

On Fri, Jul 04, 2025 at 11:08:04AM -0700, Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> More specifically the rstat updater and the flusher can race and cause
> a scenario where the stats updater skips adding the css to the
> lockless list but the flusher might not see those updates done by the
> skipped updater.  This is benign race and the subsequent flusher will
> flush those stats and at the moment there aren't any rstat users which
> are not fine with this kind of race.

Yes, the updaters and flushers has always been in a race.

> However some future user might want more stricter guarantee, so let's
> add appropriate comments to ease the job of future users.

I believe there should never be such (external) users as updaters
shouldn't be excluded by flushers in principle. (There is the exclusion
for bookkeeping the updated tree paths correctly though.)

So, thanks for the comments, one should be wary of them when considering
the update updated subtree.

Michal

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