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Date:   Mon, 23 Nov 2020 17:08:23 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Marco Elver <elver@google.com>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>,
        Will Deacon <will@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] kcsan: Avoid scheduler recursion by using
 non-instrumented preempt_{disable,enable}()
Message-ID: <20201123160823.GC2414@hirez.programming.kicks-ass.net>
References: <20201123132300.1759342-1-elver@google.com>
 <20201123135512.GM3021@hirez.programming.kicks-ass.net>
 <CANpmjNPwuq8Hph3oOyJCVgWQ_d-gOTPEOT3BpbR2pnm5LBeJbw@mail.gmail.com>
 <20201123155746.GA2203226@elver.google.com>
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On Mon, Nov 23, 2020 at 04:57:46PM +0100, Marco Elver wrote:
> Let me know what you prefer.
> 

> @@ -288,27 +288,19 @@ static u32 kcsan_prandom_u32_max(u32 ep_ro)
>  	u32 res;
>  
>  	/*
> +	 * Avoid recursion with scheduler by disabling KCSAN because
> +	 * preempt_enable_notrace() will still call into scheduler code.
>  	 */
> +	kcsan_disable_current();
>  	preempt_disable_notrace();
>  	state = raw_cpu_ptr(&kcsan_rand_state);
>  	res = prandom_u32_state(state);
> +	preempt_enable_notrace();
> +	kcsan_enable_current_nowarn();
>  
>  	return (u32)(((u64) res * ep_ro) >> 32);
>  }

This is much preferred over the other. The thing with _no_resched is that
you can miss a preemption for an unbounded amount of time, which is bad.

The _only_ valid use of _no_resched is when there's a call to schedule()
right after it.