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    Subject[PATCH 4.14 121/222] xen: make xen_qlock_wait() nestable
    Date
    4.14-stable review patch.  If anyone has any objections, please let me know.

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

    From: Juergen Gross <jgross@suse.com>

    commit a856531951dc8094359dfdac21d59cee5969c18e upstream.

    xen_qlock_wait() isn't safe for nested calls due to interrupts. A call
    of xen_qlock_kick() might be ignored in case a deeper nesting level
    was active right before the call of xen_poll_irq():

    CPU 1: CPU 2:
    spin_lock(lock1)
    spin_lock(lock1)
    -> xen_qlock_wait()
    -> xen_clear_irq_pending()
    Interrupt happens
    spin_unlock(lock1)
    -> xen_qlock_kick(CPU 2)
    spin_lock_irqsave(lock2)
    spin_lock_irqsave(lock2)
    -> xen_qlock_wait()
    -> xen_clear_irq_pending()
    clears kick for lock1
    -> xen_poll_irq()
    spin_unlock_irq_restore(lock2)
    -> xen_qlock_kick(CPU 2)
    wakes up
    spin_unlock_irq_restore(lock2)
    IRET
    resumes in xen_qlock_wait()
    -> xen_poll_irq()
    never wakes up

    The solution is to disable interrupts in xen_qlock_wait() and not to
    poll for the irq in case xen_qlock_wait() is called in nmi context.

    Cc: stable@vger.kernel.org
    Cc: Waiman.Long@hp.com
    Cc: peterz@infradead.org
    Signed-off-by: Juergen Gross <jgross@suse.com>
    Reviewed-by: Jan Beulich <jbeulich@suse.com>
    Signed-off-by: Juergen Gross <jgross@suse.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/xen/spinlock.c | 24 ++++++++++--------------
    1 file changed, 10 insertions(+), 14 deletions(-)

    --- a/arch/x86/xen/spinlock.c
    +++ b/arch/x86/xen/spinlock.c
    @@ -40,29 +40,25 @@ static void xen_qlock_kick(int cpu)
    */
    static void xen_qlock_wait(u8 *byte, u8 val)
    {
    + unsigned long flags;
    int irq = __this_cpu_read(lock_kicker_irq);

    /* If kicker interrupts not initialized yet, just spin */
    - if (irq == -1)
    + if (irq == -1 || in_nmi())
    return;

    - /* If irq pending already clear it and return. */
    + /* Guard against reentry. */
    + local_irq_save(flags);
    +
    + /* If irq pending already clear it. */
    if (xen_test_irq_pending(irq)) {
    xen_clear_irq_pending(irq);
    - return;
    + } else if (READ_ONCE(*byte) == val) {
    + /* Block until irq becomes pending (or a spurious wakeup) */
    + xen_poll_irq(irq);
    }

    - if (READ_ONCE(*byte) != val)
    - return;
    -
    - /*
    - * If an interrupt happens here, it will leave the wakeup irq
    - * pending, which will cause xen_poll_irq() to return
    - * immediately.
    - */
    -
    - /* Block until irq becomes pending (or perhaps a spurious wakeup) */
    - xen_poll_irq(irq);
    + local_irq_restore(flags);
    }

    static irqreturn_t dummy_handler(int irq, void *dev_id)

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