Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup

From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Gaurav Kohli <gkohli@codeaurora.org>,
	tglx@linutronix.de, mpe@ellerman.id.au, mingo@kernel.org,
	bigeasy@linutronix.de, linux-kernel@vger.kernel.org,
	linux-arm-msm@vger.kernel.org,
	Neeraj Upadhyay <neeraju@codeaurora.org>,
	Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup
Date: Thu, 26 Apr 2018 18:18:20 +0200	[thread overview]
Message-ID: <20180426161820.GA15391@redhat.com> (raw)
In-Reply-To: <20180426084131.GV4129@hirez.programming.kicks-ass.net>

On 04/26, Peter Zijlstra wrote:
>
> For the others, I think we want to do something like the below. I still
> need to look at TASK_TRACED, which I suspect is also special,

Yes, and TASK_STOPPED.

ptrace_freeze_traced() and ptrace_unfreeze_traced() should be fine, but
ptrace_stop() wants set_special_state() too, I think.

> but ptrace always hurts my brain.

mine too ;)

> +/*
> + * set_special_state() should be used for those states when the blocking task
> + * can not use the regular condition based wait-loop. In that case we must
> + * serialize against wakeups such that any possible in-flight TASK_RUNNING stores
> + * will not collide with out state change.
> + */
> +#define set_special_state(state_value)					\
> +	do {								\
> +		unsigned long flags; /* may shadow */			\
> +		raw_spin_lock_irqsave(&current->pi_lock, flags);	\
> +		current->state = (state_value);				\
> +		raw_spin_unlock_irqrestore(&current->pi_lock, flags);	\
> +	} while (0)
> +

Agreed.

I thought that perhaps we can change ttwu_do_wakeup() cmpxchg() instead of
plain p->state = TASK_RUNNING, but this helper looks much more clear and simple.

Oleg.