[Xen-devel] [PATCH v12 07/11] qspinlock: Revert to test-and-set on hypervisors

[Waiman Long <Waiman.Long@xxxxxx>]

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

When we detect a hypervisor (!paravirt, see qspinlock paravirt support
patches), revert to a simple test-and-set lock to avoid the horrors
of queue preemption.

Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
 arch/x86/include/asm/qspinlock.h |   14 ++++++++++++++
 include/asm-generic/qspinlock.h  |    7 +++++++
 kernel/locking/qspinlock.c       |    3 +++
 3 files changed, 24 insertions(+), 0 deletions(-)

diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index a6a8762..05a77fe 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -1,6 +1,7 @@
 #ifndef _ASM_X86_QSPINLOCK_H
 #define _ASM_X86_QSPINLOCK_H
 
+#include <asm/cpufeature.h>
 #include <asm-generic/qspinlock_types.h>
 
 #ifndef CONFIG_X86_PPRO_FENCE
@@ -20,6 +21,19 @@ static inline void queue_spin_unlock(struct qspinlock *lock)
 
 #endif /* !CONFIG_X86_PPRO_FENCE */
 
+#define virt_queue_spin_lock virt_queue_spin_lock
+
+static inline bool virt_queue_spin_lock(struct qspinlock *lock)
+{
+       if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
+               return false;
+
+       while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0)
+               cpu_relax();
+
+       return true;
+}
+
 #include <asm-generic/qspinlock.h>
 
 #endif /* _ASM_X86_QSPINLOCK_H */
diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
index e8a7ae8..a53a7bb 100644
--- a/include/asm-generic/qspinlock.h
+++ b/include/asm-generic/qspinlock.h
@@ -98,6 +98,13 @@ static __always_inline void queue_spin_unlock(struct 
qspinlock *lock)
 }
 #endif
 
+#ifndef virt_queue_spin_lock
+static __always_inline bool virt_queue_spin_lock(struct qspinlock *lock)
+{
+       return false;
+}
+#endif
+
 /*
  * Initializier
  */
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index fb0e988..1c1926a 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -257,6 +257,9 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 
val)
 
        BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
 
+       if (virt_queue_spin_lock(lock))
+               return;
+
        /*
         * wait for in-progress pending->locked hand-overs
         *
-- 
1.7.1


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