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@baidu.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