Re: [tip:sched/core] sched/numa: Introduce migrate_swap()

[Mel Gorman]

On Thu, Oct 10, 2013 at 08:17:22PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 09, 2013 at 10:30:13AM -0700, tip-bot for Peter Zijlstra wrote:
> > sched/numa: Introduce migrate_swap()
> 
> Thanks to Rik for writing the Changelog!
> 
> ---
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Subject: sched: Fix race in migrate_swap_stop
> 
> There is a subtle race in migrate_swap, when task P, on CPU A, decides to swap
> places with task T, on CPU B.
> 
> Task P:
>   - call migrate_swap
> Task T:
>   - go to sleep, removing itself from the runqueue
> Task P:
>   - double lock the runqueues on CPU A & B
> Task T:
>   - get woken up, place itself on the runqueue of CPU C
> Task P:
>   - see that task T is on a runqueue, and pretend to remove it
>     from the runqueue on CPU B
> 
> Now CPUs B & C both have corrupted scheduler data structures.
> 
> This patch fixes it, by holding the pi_lock for both of the tasks
> involved in the migrate swap. This prevents task T from waking up,
> and placing itself onto another runqueue, until after migrate_swap
> has released all locks.
> 
> This means that, when migrate_swap checks, task T will be either
> on the runqueue where it was originally seen, or not on any
> runqueue at all. Migrate_swap deals correctly with of those cases.
> 
> Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Tested-by: Joe Mario <jmario@xxxxxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxx>

-- 
Mel Gorman
SUSE Labs
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