linux-kernel - Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK

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Message-Id: <5DA6D217-8847-4760-9C23-CB1B26B5CC2B@joelfernandes.org>
Date:   Wed, 12 Jul 2023 22:16:34 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc:     Sandeep Dhavale <dhavale@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        linux-erofs@...ts.ozlabs.org, xiang@...nel.org,
        Will Shiu <Will.Shiu@...iatek.com>, kernel-team@...roid.com,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC



> On Jul 12, 2023, at 10:02 PM, Gao Xiang <hsiangkao@...ux.alibaba.com> wrote:
> 
> 
> 
>> On 2023/7/13 08:32, Joel Fernandes wrote:
>> On Wed, Jul 12, 2023 at 02:20:56PM -0700, Sandeep Dhavale wrote:
>> [..]
>>>> As such this patch looks correct to me, one thing I noticed is that
>>>> you can check rcu_is_watching() like the lockdep-enabled code does.
>>>> That will tell you also if a reader-section is possible because in
>>>> extended-quiescent-states, RCU readers should be non-existent or
>>>> that's a bug.
>>>> 
>>> Please correct me if I am wrong, reading from the comment in
>>> kernel/rcu/update.c rcu_read_lock_held_common()
>>> ..
>>>   * The reason for this is that RCU ignores CPUs that are
>>>  * in such a section, considering these as in extended quiescent state,
>>>  * so such a CPU is effectively never in an RCU read-side critical section
>>>  * regardless of what RCU primitives it invokes.
>>> 
>>> It seems rcu will treat this as lock not held rather than a fact that
>>> lock is not held. Is my understanding correct?
>> If RCU treats it as a lock not held, that is a fact for RCU ;-). Maybe you
>> mean it is not a fact for erofs?
> 
> I'm not sure if I get what you mean, EROFS doesn't take any RCU read lock

We are discussing the case 3 you mentioned below.

> here:
> 
> z_erofs_decompressqueue_endio() is actually a "bio->bi_end_io", previously
> which can be called under two scenarios:
> 
> 1) under softirq context, which is actually part of device I/O compleltion;
> 
> 2) under threaded context, like what dm-verity or likewise calls.
> 
> But EROFS needs to decompress in a threaded context anyway, so we trigger
> a workqueue to resolve the case 1).
> 
> 
> Recently, someone reported there could be some case 3) [I think it was
> introduced recently but I have no time to dig into it]:
> 
> case 3: under RCU read lock context, which is shown by this:
> https://lore.kernel.org/r/4a8254eb-ac39-1e19-3d82-417d3a7b9f94@linux.alibaba.com/T/#u
> 
> and such RCU read lock is taken in __blk_mq_run_dispatch_ops().
> 
> But as the commit shown, we only need to trigger a workqueue for case 1)
> and 3) due to performance reasons.
> 
> Hopefully I show it more clear.

Makes sense. Thanks,

 - Joel

> 
> Thanks,
> Gao Xiang