linux-kernel - Re: [PATCH V3 1/1] tick: broadcast-hrtimer: Fix a race in bc_set

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Message-ID: <20190926154356.GA29221@vmlxhi-102.adit-jv.com>
Date:   Thu, 26 Sep 2019 17:43:56 +0200
From:   Eugeniu Rosca <erosca@...adit-jv.com>
To:     Balasubramani Vivekanandan <balasubramani_vivekanandan@...tor.com>
CC:     <fweisbec@...il.com>, <tglx@...utronix.de>, <mingo@...nel.org>,
        <peterz@...radead.org>, <linux-renesas-soc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Eugeniu Rosca <erosca@...adit-jv.com>,
        Eugeniu Rosca <roscaeugeniu@...il.com>
Subject: Re: [PATCH V3 1/1] tick: broadcast-hrtimer: Fix a race in bc_set_next

On Thu, Sep 26, 2019 at 03:51:01PM +0200, Balasubramani Vivekanandan wrote:
> When a cpu requests broadcasting, before starting the tick broadcast
> hrtimer, bc_set_next() checks if the timer callback (bc_handler) is
> active using hrtimer_try_to_cancel(). But hrtimer_try_to_cancel() does
> not provide the required synchronization when the callback is active on
> other core.

[..]

> diff --git a/kernel/time/tick-broadcast-hrtimer.c b/kernel/time/tick-broadcast-hrtimer.c
> index c1f5bb590b5e..f070f9734792 100644
> --- a/kernel/time/tick-broadcast-hrtimer.c
> +++ b/kernel/time/tick-broadcast-hrtimer.c

[..]

FWIW, the patch seems to fix the very first commit adding hrtimer
broadcast, i.e. v3.15-rc1 commit 5d1638acb9f62f ("tick: Introduce
hrtimer based broadcast"), so maybe adding a Fixes: tag could be
relevant/beneficial for the stable trees?

-- 
Best Regards,
Eugeniu