Re: rcu_preempt caused oom

[Paul E. McKenney]

On Mon, Dec 17, 2018 at 03:15:42AM +0000, He, Bo wrote:
> for double confirm the issue is not reproduce after 90 hours, we tried only add the enclosed patch on the easy reproduced build, the issue is not reproduced after 63 hours in the whole weekend on 16 boards.
> so current conclusion is the debug patch has extreme  effect on the rcu issue.

This is not a surprise.  (Please see the end of this email for a
replacement patch that won't suppress the bug.)

To see why this is not a surprise, let's take a closer look at your patch,
in light of the comment header for wait_event_idle_timeout_exclusive():

 * Returns:
 * 0 if the @condition evaluated to %false after the @timeout elapsed,
 * 1 if the @condition evaluated to %true after the @timeout elapsed,
 * or the remaining jiffies (at least 1) if the @condition evaluated
 * to %true before the @timeout elapsed.

The situation we are seeing is that the RCU_GP_FLAG_INIT is set, but
the rcu_preempt task does not wake up.  This would correspond to
the second case above, that is, a return value of 1.  Looking now
at your patch, with comments interspersed below:

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