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Subject[PATCH 8/8] xen/pvticketlock: allow interrupts to be enabled while blocking
Date
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

If interrupts were enabled when taking the spinlock, we can leave them
enabled while blocking to get the lock.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
---
arch/x86/xen/spinlock.c | 42 +++++++++++++++++++++++++++++++++++-------
1 files changed, 35 insertions(+), 7 deletions(-)
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index c939723..d2335f88 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -106,11 +106,28 @@ static void xen_lock_spinning(struct arch_spinlock *lock, __ticket_t want)

start = spin_time_start();

- /* Make sure interrupts are disabled to ensure that these
- per-cpu values are not overwritten. */
+ /*
+ * Make sure an interrupt handler can't upset things in a
+ * partially setup state.
+ */
local_irq_save(flags);

+ /*
+ * We don't really care if we're overwriting some other
+ * (lock,want) pair, as that would mean that we're currently
+ * in an interrupt context, and the outer context had
+ * interrupts enabled. That has already kicked the VCPU out
+ * of xen_poll_irq(), so it will just return spuriously and
+ * retry with newly setup (lock,want).
+ *
+ * The ordering protocol on this is that the "lock" pointer
+ * may only be set non-NULL if the "want" ticket is correct.
+ * If we're updating "want", we must first clear "lock".
+ */
+ w->lock = NULL;
+ smp_wmb();
w->want = want;
+ smp_wmb();
w->lock = lock;

/* This uses set_bit, which atomic and therefore a barrier */
@@ -124,21 +141,30 @@ static void xen_lock_spinning(struct arch_spinlock *lock, __ticket_t want)
/* Only check lock once pending cleared */
barrier();

- /* Mark entry to slowpath before doing the pickup test to make
- sure we don't deadlock with an unlocker. */
+ /*
+ * Mark entry to slowpath before doing the pickup test to make
+ * sure we don't deadlock with an unlocker.
+ */
__ticket_enter_slowpath(lock);

- /* check again make sure it didn't become free while
- we weren't looking */
+ /*
+ * check again make sure it didn't become free while
+ * we weren't looking
+ */
if (ACCESS_ONCE(lock->tickets.head) == want) {
ADD_STATS(taken_slow_pickup, 1);
goto out;
}

+ /* Allow interrupts while blocked */
+ local_irq_restore(flags);
+
/* Block until irq becomes pending (or perhaps a spurious wakeup) */
xen_poll_irq(irq);
ADD_STATS(taken_slow_spurious, !xen_test_irq_pending(irq));

+ local_irq_save(flags);
+
kstat_incr_irqs_this_cpu(irq, irq_to_desc(irq));

out:
@@ -160,7 +186,9 @@ static void xen_unlock_kick(struct arch_spinlock *lock, __ticket_t next)
for_each_cpu(cpu, &waiting_cpus) {
const struct xen_lock_waiting *w = &per_cpu(lock_waiting, cpu);

- if (w->lock == lock && w->want == next) {
+ /* Make sure we read lock before want */
+ if (ACCESS_ONCE(w->lock) == lock &&
+ ACCESS_ONCE(w->want) == next) {
ADD_STATS(released_slow_kicked, 1);
xen_send_IPI_one(cpu, XEN_SPIN_UNLOCK_VECTOR);
break;
--
1.7.6


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