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

From: Pavel Machek
Date: Sun Oct 13 2019 - 11:48:37 EST

On Thu 2019-10-10 10:35:39, Greg Kroah-Hartman wrote:
> From: Li RongQing <lirongqing@xxxxxxxxx>
> 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,
(cesky, pictures)

Attachment: signature.asc
Description: Digital signature