linux-kernel - [PATCH v2] memcg: Remove the lockdep assert from __mod_objcg

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Message-ID: <20240528141341.rz_rytN_@linutronix.de>
Date: Tue, 28 May 2024 16:13:41 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: "Vlastimil Babka (SUSE)" <vbabka@...nel.org>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH v2] memcg: Remove the lockdep assert from
 __mod_objcg_mlstate().

The assert was introduced in the commit cited below as an insurance that
the semantic is the same after the local_irq_save() has been removed and
the function has been made static.

The original requirement to disable interrupt was due the modification
of per-CPU counters which require interrupts to be disabled because the
counter update operation is not atomic and some of the counters are
updated from interrupt context.

All callers of __mod_objcg_mlstate() acquire a lock
(memcg_stock.stock_lock) which disables interrupts on !PREEMPT_RT and
the lockdep assert is satisfied. On PREEMPT_RT the interrupts are not
disabled and the assert triggers.

The safety of the counter update is already ensured by
VM_WARN_ON_IRQS_ENABLED() which is part of __mod_memcg_lruvec_state() and
does not require yet another check.

Remove the lockdep assert from __mod_objcg_mlstate().

Fixes: 91882c1617c15 ("memcg: simple cleanup of stats update functions")
Link: https://lore.kernel.org/r/20240528121928.i-Gu7Jvg@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
On 2024-05-28 15:44:51 [+0200], Vlastimil Babka (SUSE) wrote:
> I think just s/memcg_stats_lock()/__mod_memcg_lruvec_state()/ in your
> phrasing, since we are removing the lockdep assert from path that calls
> __mod_memcg_lruvec_state() and not memcg_stats_lock()?
> Or am I missing something?

Yeah, makes sense.

 mm/memcontrol.c |    2 --
 1 file changed, 2 deletions(-)

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3147,8 +3147,6 @@ static inline void __mod_objcg_mlstate(s
 	struct mem_cgroup *memcg;
 	struct lruvec *lruvec;

-	lockdep_assert_irqs_disabled();
-
 	rcu_read_lock();
 	memcg = obj_cgroup_memcg(objcg);
 	lruvec = mem_cgroup_lruvec(memcg, pgdat);