linux-kernel - Re: [PATCH REPOST 2/2] signal: Don't disable preemption in ptrace_stop() on PREEMPT

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Date:   Tue, 6 Jun 2023 13:04:48 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH REPOST 2/2] signal: Don't disable preemption in
 ptrace_stop() on PREEMPT_RT.

The patch LGTM, but I am a bit confused by the changelog/comments,
I guess I missed something...

On 06/06, Sebastian Andrzej Siewior wrote:
>
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2328,11 +2328,16 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
>  	 * The preempt-disable section ensures that there will be no preemption
>  	 * between unlock and schedule() and so improving the performance since
>  	 * the ptracer has no reason to sleep.
> +	 *
> +	 * This optimisation is not doable on PREEMPT_RT due to the spinlock_t
> +	 * within the preempt-disable section.
>  	 */
> -	preempt_disable();
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +		preempt_disable();

Not only we the problems with cgroup_enter_frozen(), afaics (please correct me)
this optimisation doesn't work on RT anyway?

IIUC, read_lock() on RT disables migration but not preemption, so it is simply
too late to do preempt_disable() before unlock/schedule. The tracer can preempt
the tracee right after do_notify_parent_cldstop().

Oleg.