4.4-stable review patch. If anyone has any objections, please let me know.
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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 @@ -41,29 +41,25 @@ 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); }
#else /* CONFIG_QUEUED_SPINLOCKS */