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

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Date: Tue, 28 May 2024 14:34:55 +0200
From: "Vlastimil Babka (SUSE)" <vbabka@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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: Re: [PATCH] memcg: Remove the lockdep assert from
 __mod_objcg_mlstate().

On 5/28/24 2:19 PM, Sebastian Andrzej Siewior wrote:
> 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 memcg_stats_lock() and does
> not require yet another check.

I think here it's __mod_memcg_lruvec_state() doing the VM_WARN_ON_ as we
don't go through memcg_stats_lock()?

> Remove the lockdep assert from __mod_objcg_mlstate().
> 
> Fixes: 91882c1617c15 ("memcg: simple cleanup of stats update functions")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

mm-hotfixes as it's a rc1 regression

> ---
>  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);