ACK: [PATCH][SRU][ZESTY] UBUNTU:SAUCE: exec: fix lockup because retry loop may never exit

[Seth Forshee]

On Thu, Dec 07, 2017 at 05:31:34PM +0000, Colin King wrote:
> From: Colin Ian King <colin.king at canonical.com>
> 
> BugLink: https://launchpad.net/bugs/1730717
> 
> My early fix for bug LP#1672819 could get stuck in an infinite
> retry loop causing lockups.  Currently the retry is aggressively
> spinning on a check and will never abort.  Relax the aggressive
> retry by adding a small interruptible delay and limit the number
> of retries.
> 
> I've analyzed the retries and 95% of the cases on an 8 CPU Xeon
> host never hit the retry loop and less that 0.5% of retries are
> ever more than 1 retry using the original reproducer program.
> Considering that the reproducer is an extreeme case of forcing
> the race condition I believe we now have a suitable balance of
> non-aggressive CPU eating retries and a mechanism to bail out
> after enough retries and avoid a lockup.
> 
> Admittedly this workaround is a bit of an ugly hack, but the
> retry path is never executed for the majority of use cases and
> only hits the retry/delay for the racy condition we hit with the
> original bug.
> 
> Signed-off-by: Colin Ian King <colin.king at canonical.com>

Looks reasonable. I have tested this (in artful) to confirm that it
fixes the regression.

Acked-by: Seth Forshee <seth.forshee at canonical.com>