linux-kernel - Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall

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Message-ID: <87zg2hcufn.ffs@tglx>
Date:   Wed, 23 Aug 2023 23:36:44 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Z qiang <qiang.zhang1211@...il.com>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     paulmck@...nel.org, Huacai Chen <chenhuacai@...ngson.cn>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        John Stultz <jstultz@...gle.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Binbin Zhou <zhoubinbin@...ngson.cn>
Subject: Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset()

On Wed, Aug 16 2023 at 18:06, Z. qiang wrote:
>> On Wed, Aug 16, 2023 at 1:09 PM Z qiang <qiang.zhang1211@...il.com> wrote:
>> > You can replace rcu_state.jiffies_stall update by setting rcu_cpu_stall_suppress
>> > in rcu_cpu_stall_reset(),  and reset rcu_cpu_stall_suppress in rcu_gp_init() and
>> > rcu_gp_cleanup().
>> What's the advantage compared with updating jiffies? Updating jiffies
>> seems more straight forward.
>>
>
> In do_update_jiffies_64(), need to acquire jiffies_lock raw spinlock,
> like you said, kgdb is not the only caller of rcu_cpu_stall_reset(),
> the rcu_cpu_stall_reset() maybe invoke in NMI  (arch/x86/platform/uv/uv_nmi.c)

What's worse is that KGDB can set breakpoints pretty much everywhere and
there is no guarantee that the jiffies lock is not held when a
breakpoint hits.

Thanks,

        tglx