linux-kernel - Re: [PATCH next v3 2/2] printk: fix cpu lock ordering

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Date:   Wed, 16 Jun 2021 13:30:34 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH next v3 2/2] printk: fix cpu lock ordering

On Tue 2021-06-15 19:55:47, John Ogness wrote:
> The cpu lock implementation uses a full memory barrier to take
> the lock, but no memory barriers when releasing the lock. This
> means that changes performed by a lock owner may not be seen by
> the next lock owner. This may have been "good enough" for use
> by dump_stack() as a serialization mechanism, but it is not
> enough to provide proper protection for a critical section.
> 
> Correct this problem by using acquire/release memory barriers
> for lock/unlock, respectively.
> 
> Note that it is not necessary for a cpu lock to disable
> interrupts. However, in upcoming work this cpu lock will be used
> for emergency tasks (for example, atomic consoles during kernel
> crashes) and any interruptions while holding the cpu lock should
> be avoided if possible.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

The patch looks good to me, except for the problems already
reported for the 1st patch.

Best Regards,
Petr