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.

Tested-by: Joe Mario <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

0 files changed, 0 insertions, 0 deletions