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    Subject[PATCH 4.18 040/123] stop_machine: Atomically queue and wake stopper threads
    Date
    4.18-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Prasad Sodagudi <psodagud@codeaurora.org>

    commit cfd355145c32bb7ccb65fccbe2d67280dc2119e1 upstream.

    When cpu_stop_queue_work() releases the lock for the stopper
    thread that was queued into its wake queue, preemption is
    enabled, which leads to the following deadlock:

    CPU0 CPU1
    sched_setaffinity(0, ...)
    __set_cpus_allowed_ptr()
    stop_one_cpu(0, ...) stop_two_cpus(0, 1, ...)
    cpu_stop_queue_work(0, ...) cpu_stop_queue_two_works(0, ..., 1, ...)

    -grabs lock for migration/0-
    -spins with preemption disabled,
    waiting for migration/0's lock to be
    released-

    -adds work items for migration/0
    and queues migration/0 to its
    wake_q-

    -releases lock for migration/0
    and preemption is enabled-

    -current thread is preempted,
    and __set_cpus_allowed_ptr
    has changed the thread's
    cpu allowed mask to CPU1 only-

    -acquires migration/0 and migration/1's
    locks-

    -adds work for migration/0 but does not
    add migration/0 to wake_q, since it is
    already in a wake_q-

    -adds work for migration/1 and adds
    migration/1 to its wake_q-

    -releases migration/0 and migration/1's
    locks, wakes migration/1, and enables
    preemption-

    -since migration/1 is requested to run,
    migration/1 begins to run and waits on
    migration/0, but migration/0 will never
    be able to run, since the thread that
    can wake it is affine to CPU1-

    Disable preemption in cpu_stop_queue_work() before queueing works for
    stopper threads, and queueing the stopper thread in the wake queue, to
    ensure that the operation of queueing the works and waking the stopper
    threads is atomic.

    Fixes: 0b26351b910f ("stop_machine, sched: Fix migrate_swap() vs. active_balance() deadlock")
    Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>
    Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Cc: peterz@infradead.org
    Cc: matt@codeblueprint.co.uk
    Cc: bigeasy@linutronix.de
    Cc: gregkh@linuxfoundation.org
    Cc: stable@vger.kernel.org
    Link: https://lkml.kernel.org/r/1533329766-4856-1-git-send-email-isaacm@codeaurora.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    Co-Developed-by: Isaac J. Manjarres <isaacm@codeaurora.org>

    ---
    kernel/stop_machine.c | 2 ++
    1 file changed, 2 insertions(+)

    --- a/kernel/stop_machine.c
    +++ b/kernel/stop_machine.c
    @@ -81,6 +81,7 @@ static bool cpu_stop_queue_work(unsigned
    unsigned long flags;
    bool enabled;

    + preempt_disable();
    raw_spin_lock_irqsave(&stopper->lock, flags);
    enabled = stopper->enabled;
    if (enabled)
    @@ -90,6 +91,7 @@ static bool cpu_stop_queue_work(unsigned
    raw_spin_unlock_irqrestore(&stopper->lock, flags);

    wake_up_q(&wakeq);
    + preempt_enable();

    return enabled;
    }

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