linux-kernel - Re: [PATCH v2] sched,livepatch: Untangle cond

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Message-ID: <alpine.LSU.2.21.2505131529080.19621@pobox.suse.cz>
Date: Tue, 13 May 2025 15:34:50 +0200 (CEST)
From: Miroslav Benes <mbenes@...e.cz>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
cc: linux-kernel@...r.kernel.org, live-patching@...r.kernel.org, 
    Peter Zijlstra <peterz@...radead.org>, 
    Josh Poimboeuf <jpoimboe@...hat.com>, mingo@...nel.com, 
    juri.lelli@...hat.com, vincent.guittot@...aro.org, 
    dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, 
    mgorman@...e.de, vschneid@...hat.com, jpoimboe@...nel.org, 
    jikos@...nel.org, pmladek@...e.com, joe.lawrence@...hat.com, 
    Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] sched,livepatch: Untangle cond_resched() and
 live-patching

Hi,

thanks for the updated version.

On Fri, 9 May 2025, Sebastian Andrzej Siewior wrote:

> From: Peter Zijlstra <peterz@...radead.org>
> 
> With the goal of deprecating / removing VOLUNTARY preempt, live-patch
> needs to stop relying on cond_resched() to make forward progress.
> 
> Instead, rely on schedule() with TASK_FREEZABLE set. Just like
> live-patching, the freezer needs to be able to stop tasks in a safe /
> known state.
> 
> Compile tested only.

livepatch selftests pass and I also ran some more.
 
> [bigeasy: use likely() in __klp_sched_try_switch() and update comments]
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Acked-by: Miroslav Benes <mbenes@...e.cz>

A nit below if there is an another version, otherwise Petr might fix it 
when merging.

> @@ -365,27 +356,20 @@ static bool klp_try_switch_task(struct task_struct *task)
>  
>  void __klp_sched_try_switch(void)
>  {
> +	/*
> +	 * This function is called from __schedule() while a context switch is
> +	 * about to happen. Preemption is already disabled and klp_mutex
> +	 * can't be acquired.
> +	 * Disabled preemption is used to prevent racing with other callers of
> +	 * klp_try_switch_task(). Thanks to task_call_func() they won't be
> +	 * able to switch to this task while it's running.
> +	 */
> +	lockdep_assert_preemption_disabled();
> +
> +	/* Make sure current didn't get patched */
>       if (likely(!klp_patch_pending(current)))
>                return;

This last comment is not precise. If !klp_patch_pending(), there is 
nothing to do. Fast way out. So if it was up to me, I would remove the 
line all together.

Miroslav