Commit a8565319 authored Oct 01, 2018 by Juergen Gross

xen: make xen_qlock_wait() nestable



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>

parent 2ac2a7d4

arch/x86/xen/spinlock.c

+10 −14

Original line number	Diff line number	Diff line
		@@ -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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