From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: sashiko-reviews@lists.linux.dev, David Hildenbrand <david@kernel.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
Masami Hiramatsu <mhiramat@kernel.org>,
Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
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Martin Liu <liumartin@google.com>,
David Rientjes <rientjes@google.com>,
christian.koenig@amd.com, Shakeel Butt <shakeel.butt@linux.dev>,
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linux-mm@kvack.org, Jonathan Corbet <corbet@lwn.net>,
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Ingo Molnar <mingo@redhat.com>,
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Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
Valentin Schneider <vschneid@redhat.com>,
K Prateek Nayak <kprateek.nayak@amd.com>,
Kees Cook <kees@kernel.org>,
linux-trace-kernel@vger.kernel.org, Jan Kara <jack@suse.cz>,
Oleg Nesterov <oleg@redhat.com>
Subject: Re: [RFC PATCH v24 06/10] lib: percpu_counter_tree: add lazy tree activation
Date: Mon, 9 Nov 2026 14:40:30 -0500 [thread overview]
Message-ID: <c18938e0-3588-4141-8974-eae43697c531@efficios.com> (raw)
In-Reply-To: <sashiko-outbox-202244@kernel.org>
On 2026-11-06 15:37, sashiko-bot@kernel.org wrote:
> [Severity: Low]
> In hpcc_test_lazy_activation_race(), can a weakly ordered CPU reorder the
> relaxed WRITE_ONCE(sh.go, true) before the loads of pct.bias and
> pct.atomic_bias inside check_lazy_counter()?
[...]
> Should sh.go be set with smp_store_release(&sh.go, true) (or preceded by
> smp_mb()) so the pre-activation reads in check_lazy_counter() complete before
> the paused workers resume modifying pct?
Yes, this is a bug in the test. Nothing orders the reads of the counter
in check_lazy_counter() before the store to sh.go. On arm64 or POWER, a
worker can see go set and update the counter before those reads
complete. The check then fails, although the library is correct. x86
does not reorder a load with a later store, so our test runs could not
hit it.
Planned fix for the next round: the test sets go with
smp_store_release(), and the workers read it with smp_load_acquire().
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
next prev parent reply other threads:[~2026-11-09 19:40 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-05 20:36 [RFC PATCH v24 00/10] Hierarchical Percpu Counters for RSS Mathieu Desnoyers
2026-11-05 20:36 ` [RFC PATCH v24 01/10] lib: introduce hierarchical per-cpu counters Mathieu Desnoyers
2026-11-05 20:36 ` [RFC PATCH v24 02/10] lib: test " Mathieu Desnoyers
2026-11-05 20:36 ` [RFC PATCH v24 03/10] mm: use hierarchical per-cpu counters for the RSS counters Mathieu Desnoyers
2026-11-05 20:36 ` [RFC PATCH v24 04/10] tools/testing/hpcc: add userspace test of hierarchical per-CPU counters Mathieu Desnoyers
2026-11-05 20:36 ` [RFC PATCH v24 05/10] mm: report the approximate RSS in the rss_stat tracepoint Mathieu Desnoyers
2026-11-05 20:36 ` [RFC PATCH v24 06/10] lib: percpu_counter_tree: add lazy tree activation Mathieu Desnoyers
2026-11-06 20:37 ` sashiko-bot
2026-11-09 19:40 ` Mathieu Desnoyers [this message]
2026-11-05 20:36 ` [RFC PATCH v24 07/10] mm: keep the RSS counters of single-threaded processes exact Mathieu Desnoyers
2026-11-05 20:36 ` [RFC PATCH v24 08/10] lib: percpu_counter_tree: add the arity boot parameter Mathieu Desnoyers
2026-11-05 20:36 ` [RFC PATCH v24 09/10] mm: add the rss_stat_batch " Mathieu Desnoyers
2026-11-05 20:36 ` [RFC PATCH v24 10/10] lib: add a benchmark of hierarchical per-cpu counters Mathieu Desnoyers
2026-11-06 20:37 ` sashiko-bot
2026-11-09 19:41 ` Mathieu Desnoyers
2026-11-09 20:35 ` [RFC PATCH v24 00/10] Hierarchical Percpu Counters for RSS David Hildenbrand (Arm)
2026-11-11 1:47 ` Mathieu Desnoyers
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