• Peter Zijlstra's avatar
    sched: Fix race in migrate_swap_stop() · 74602315
    Peter Zijlstra authored
    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: default avatarJoe Mario <jmario@redhat.com>
    Acked-by: default avatarMel Gorman <mgorman@suse.de>
    Reviewed-by: default avatarRik van Riel <riel@redhat.com>
    Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
    Cc: hannes@cmpxchg.org
    Cc: aarcange@redhat.com
    Cc: srikar@linux.vnet.ibm.com
    Cc: tglx@linutronix.de
    Cc: hpa@zytor.com
    Link: http://lkml.kernel.org/r/20131010181722.GO13848@laptop.programming.kicks-ass.netSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    74602315
core.c 184 KB