Received-SPF: pass (google.com: domain of linux-kernel+bounces-192503-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Date: Tue, 28 May 2024 16:13:41 +0200
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: "Vlastimil Babka (SUSE)" <vbabka@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Muchun Song <muchun.song@linux.dev>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>
Subject: [PATCH v2] memcg: Remove the lockdep assert from
 __mod_objcg_mlstate().
Message-ID: <20240528141341.rz_rytN_@linutronix.de>
References: <20240528121928.i-Gu7Jvg@linutronix.de>
 <09e085bb-f09e-4901-a2dd-a0b789bb8a4d@kernel.org>
 <20240528134027.OxDASsS3@linutronix.de>
 <c84d6962-34fa-42e5-899c-925579cbfb26@kernel.org>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <c84d6962-34fa-42e5-899c-925579cbfb26@kernel.org>

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@linutronix.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);