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    SubjectRe: [tip:sched/core] sched/numa: Introduce migrate_swap()
    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@infradead.org>
    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@infradead.org>
    Tested-by: Joe Mario <jmario@redhat.com>
    ---
    kernel/sched/core.c | 4 ++++
    kernel/sched/fair.c | 9 ---------
    kernel/sched/sched.h | 18 ++++++++++++++++++
    3 files changed, 22 insertions(+), 9 deletions(-)

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 0c3feebcf112..a972acd468b0 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -1049,6 +1049,8 @@ static int migrate_swap_stop(void *data)
    src_rq = cpu_rq(arg->src_cpu);
    dst_rq = cpu_rq(arg->dst_cpu);

    + double_raw_lock(&arg->src_task->pi_lock,
    + &arg->dst_task->pi_lock);
    double_rq_lock(src_rq, dst_rq);
    if (task_cpu(arg->dst_task) != arg->dst_cpu)
    goto unlock;
    @@ -1069,6 +1071,8 @@ static int migrate_swap_stop(void *data)

    unlock:
    double_rq_unlock(src_rq, dst_rq);
    + raw_spin_unlock(&arg->dst_task->pi_lock);
    + raw_spin_unlock(&arg->src_task->pi_lock);

    return ret;
    }
    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 803e343d7c89..a60d57c5379a 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -1448,15 +1448,6 @@ static inline void put_numa_group(struct numa_group *grp)
    kfree_rcu(grp, rcu);
    }

    -static void double_lock(spinlock_t *l1, spinlock_t *l2)
    -{
    - if (l1 > l2)
    - swap(l1, l2);
    -
    - spin_lock(l1);
    - spin_lock_nested(l2, SINGLE_DEPTH_NESTING);
    -}
    -
    static void task_numa_group(struct task_struct *p, int cpupid, int flags,
    int *priv)
    {
    diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
    index d69cb325c27e..ffc708717b70 100644
    --- a/kernel/sched/sched.h
    +++ b/kernel/sched/sched.h
    @@ -1249,6 +1249,24 @@ static inline void double_unlock_balance(struct rq *this_rq, struct rq *busiest)
    lock_set_subclass(&this_rq->lock.dep_map, 0, _RET_IP_);
    }

    +static inline void double_lock(spinlock_t *l1, spinlock_t *l2)
    +{
    + if (l1 > l2)
    + swap(l1, l2);
    +
    + spin_lock(l1);
    + spin_lock_nested(l2, SINGLE_DEPTH_NESTING);
    +}
    +
    +static inline void double_raw_lock(raw_spinlock_t *l1, raw_spinlock_t *l2)
    +{
    + if (l1 > l2)
    + swap(l1, l2);
    +
    + raw_spin_lock(l1);
    + raw_spin_lock_nested(l2, SINGLE_DEPTH_NESTING);
    +}
    +
    /*
    * double_rq_lock - safely lock two runqueues
    *

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