Correcting Waiman's mail address
On 01/10/2018 09:16, Juergen Gross wrote:
> 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: sta...@vger.kernel.org
> Cc: long...@redhat.com
> Cc: pet...@infradead.org
> Signed-off-by: Juergen Gross <jgr...@suse.com>
> ---
> arch/x86/xen/spinlock.c | 24 ++++++++++--------------
> 1 file changed, 10 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> index cd210a4ba7b1..e8d880e98057 100644
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -39,29 +39,25 @@ 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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