Commit 4d576b57 authored by Jeremy Fitzhardinge's avatar Jeremy Fitzhardinge
Browse files

xen: only enable interrupts while actually blocking for spinlock 



Where possible we enable interrupts while waiting for a spinlock to
become free, in order to reduce big latency spikes in interrupt handling.

However, at present if we manage to pick up the spinlock just before
blocking, we'll end up holding the lock with interrupts enabled for a
while.  This will cause a deadlock if we recieve an interrupt in that
window, and the interrupt handler tries to take the lock too.

Solve this by shrinking the interrupt-enabled region to just around the
blocking call.

[ Impact: avoid race/deadlock when using Xen PV spinlocks ]

Reported-by: default avatar"Yang, Xiaowei" <xiaowei.yang@intel.com>
Signed-off-by: default avatarJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
parent 577eebea

arch/x86/xen/spinlock.c

+11 −8
Original line number Diff line number Diff line
@@ -187,7 +187,6 @@ static noinline int xen_spin_lock_slow(struct raw_spinlock *lock, bool irq_enabl 
	struct xen_spinlock *prev;

	int irq = __get_cpu_var(lock_kicker_irq);

	int ret;

	unsigned long flags;

	u64 start;



	/* If kicker interrupts not initialized yet, just spin */
@@ -199,16 +198,12 @@ static noinline int xen_spin_lock_slow(struct raw_spinlock *lock, bool irq_enabl 
	/* announce we're spinning */

	prev = spinning_lock(xl);



	flags = __raw_local_save_flags();

	if (irq_enable) {

		ADD_STATS(taken_slow_irqenable, 1);

		raw_local_irq_enable();

	}



	ADD_STATS(taken_slow, 1);

	ADD_STATS(taken_slow_nested, prev != NULL);



	do {

		unsigned long flags;



		/* clear pending */

		xen_clear_irq_pending(irq);
@@ -228,6 +223,12 @@ static noinline int xen_spin_lock_slow(struct raw_spinlock *lock, bool irq_enabl 
			goto out;

		}



		flags = __raw_local_save_flags();

		if (irq_enable) {

			ADD_STATS(taken_slow_irqenable, 1);

			raw_local_irq_enable();

		}



		/*

		 * Block until irq becomes pending.  If we're

		 * interrupted at this point (after the trylock but
@@ -238,13 +239,15 @@ static noinline int xen_spin_lock_slow(struct raw_spinlock *lock, bool irq_enabl 
		 * pending.

		 */

		xen_poll_irq(irq);



		raw_local_irq_restore(flags);



		ADD_STATS(taken_slow_spurious, !xen_test_irq_pending(irq));

	} while (!xen_test_irq_pending(irq)); /* check for spurious wakeups */



	kstat_incr_irqs_this_cpu(irq, irq_to_desc(irq));



out:

	raw_local_irq_restore(flags);

	unspinning_lock(xl, prev);

	spin_time_accum_blocked(start);

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