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    Subject[tip:sched/core] sched: Fix ancient race in do_exit()
    Commit-ID:  b5740f4b2cb3503b436925eb2242bc3d75cd3dfe
    Gitweb: http://git.kernel.org/tip/b5740f4b2cb3503b436925eb2242bc3d75cd3dfe
    Author: Yasunori Goto <y-goto@jp.fujitsu.com>
    AuthorDate: Tue, 17 Jan 2012 17:40:31 +0900
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Fri, 27 Jan 2012 11:55:36 +0100

    sched: Fix ancient race in do_exit()

    try_to_wake_up() has a problem which may change status from TASK_DEAD to
    TASK_RUNNING in race condition with SMI or guest environment of virtual
    machine. As a result, exited task is scheduled() again and panic occurs.

    Here is the sequence how it occurs:

    ----------------------------------+-----------------------------
    |
    CPU A | CPU B
    ----------------------------------+-----------------------------

    TASK A calls exit()....

    do_exit()

    exit_mm()
    down_read(mm->mmap_sem);

    rwsem_down_failed_common()

    set TASK_UNINTERRUPTIBLE
    set waiter.task <= task A
    list_add to sem->wait_list
    :
    raw_spin_unlock_irq()
    (I/O interruption occured)

    __rwsem_do_wake(mmap_sem)

    list_del(&waiter->list);
    waiter->task = NULL
    wake_up_process(task A)
    try_to_wake_up()
    (task is still
    TASK_UNINTERRUPTIBLE)
    p->on_rq is still 1.)

    ttwu_do_wakeup()
    (*A)
    :
    (I/O interruption handler finished)

    if (!waiter.task)
    schedule() is not called
    due to waiter.task is NULL.

    tsk->state = TASK_RUNNING

    :
    check_preempt_curr();
    :
    task->state = TASK_DEAD
    (*B)
    <--- set TASK_RUNNING (*C)

    schedule()
    (exit task is running again)
    BUG_ON() is called!
    --------------------------------------------------------

    The execution time between (*A) and (*B) is usually very short,
    because the interruption is disabled, and setting TASK_RUNNING at (*C)
    must be executed before setting TASK_DEAD.

    HOWEVER, if SMI is interrupted between (*A) and (*B),
    (*C) is able to execute AFTER setting TASK_DEAD!
    Then, exited task is scheduled again, and BUG_ON() is called....

    If the system works on guest system of virtual machine, the time
    between (*A) and (*B) may be also long due to scheduling of hypervisor,
    and same phenomenon can occur.

    By this patch, do_exit() waits for releasing task->pi_lock which is used
    in try_to_wake_up(). It guarantees the task becomes TASK_DEAD after
    waking up.

    Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>
    Acked-by: Oleg Nesterov <oleg@redhat.com>
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Link: http://lkml.kernel.org/r/20120117174031.3118.E1E9C6FF@jp.fujitsu.com
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    kernel/exit.c | 16 ++++++++++++++++
    1 files changed, 16 insertions(+), 0 deletions(-)

    diff --git a/kernel/exit.c b/kernel/exit.c
    index 294b170..4b4042f 100644
    --- a/kernel/exit.c
    +++ b/kernel/exit.c
    @@ -1038,6 +1038,22 @@ void do_exit(long code)
    if (tsk->nr_dirtied)
    __this_cpu_add(dirty_throttle_leaks, tsk->nr_dirtied);
    exit_rcu();
    +
    + /*
    + * The setting of TASK_RUNNING by try_to_wake_up() may be delayed
    + * when the following two conditions become true.
    + * - There is race condition of mmap_sem (It is acquired by
    + * exit_mm()), and
    + * - SMI occurs before setting TASK_RUNINNG.
    + * (or hypervisor of virtual machine switches to other guest)
    + * As a result, we may become TASK_RUNNING after becoming TASK_DEAD
    + *
    + * To avoid it, we have to wait for releasing tsk->pi_lock which
    + * is held by try_to_wake_up()
    + */
    + smp_mb();
    + raw_spin_unlock_wait(&tsk->pi_lock);
    +
    /* causes final put_task_struct in finish_task_switch(). */
    tsk->state = TASK_DEAD;
    tsk->flags |= PF_NOFREEZE; /* tell freezer to ignore us */

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