linux-kernel - Re: [PATCH 4.19 032/114] timer: Read jiffies once when forwarding base clk

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Message-ID: <20191013154748.GG13278@amd>
Date:   Sun, 13 Oct 2019 17:47:48 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Li RongQing <lirongqing@...du.com>,
        Liang ZhiCheng <liangzhicheng@...du.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 4.19 032/114] timer: Read jiffies once when forwarding
 base clk

On Thu 2019-10-10 10:35:39, Greg Kroah-Hartman wrote:
> From: Li RongQing <lirongqing@...du.com>
> 
> commit e430d802d6a3aaf61bd3ed03d9404888a29b9bf9 upstream.
> 
> The reason is that the code in collect_expired_timers() uses jiffies
> unprotected:
> 
>     if (next_event > jiffies)
>         base->clk = jiffies;
> 
> As the compiler is allowed to reload the value base->clk can advance
> between the check and the store and in the worst case advance farther than
> next event. That causes the timer expiry to be delayed until the wheel
> pointer wraps around.
> 
> Convert the code to use READ_ONCE()

Does it really need to use READ_ONCE? "jiffies" is already volatile,
READ_ONCE just adds another volatile...

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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