linux-kernel - [PATCH 8/8] xen/pvticketlock: allow interrupts to be enabled while blocking

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Message-Id: <b927340248ad896330903fe63a6c9adbdf533c6f.1315007226.git.jeremy.fitzhardinge@citrix.com>
Date:	Fri,  2 Sep 2011 16:54:15 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>, Andi Kleen <andi@...stfloor.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: [PATCH 8/8] xen/pvticketlock: allow interrupts to be enabled while blocking

From: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.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@...rix.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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