[Xen-changelog] [xen master] xen: arm: retry trylock if strex fails on free lock.

[patchbot@xxxxxxx]

commit 08d89ac8bd7f43d897cdffe85b236c71d3408e16
Author:     Ian Campbell <ian.campbell@xxxxxxxxxx>
AuthorDate: Fri Jul 19 16:20:10 2013 +0100
Commit:     Ian Campbell <ian.campbell@xxxxxxxxxx>
CommitDate: Thu Aug 22 15:48:13 2013 +0100

    xen: arm: retry trylock if strex fails on free lock.
    
    This comes from the Linux patches 15e7e5c1ebf5 for arm32 and 4ecf7ccb1973 
for
    arm64 by Will Deacon and Catalin Marinas respectively. The Linux commit 
message
    says:
    
        An exclusive store instruction may fail for reasons other than lock
        contention (e.g. a cache eviction during the critical section) so, in
        line with other architectures using similar exclusive instructions
        (alpha, mips, powerpc), retry the trylock operation if the lock appears
        to be free but the strex reported failure.
    
    I have observed this due to register_cpu_notifier containing:
        if ( !spin_trylock(&cpu_add_remove_lock) )
            BUG(); /* Should never fail as we are called only during boot. */
    which was spuriously failing.
    
    The ARMv8 variant is taken directly from the Linux patch. For v7 I had to
    reimplement since we don't currently use ticket locks.
    
    Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
    Acked-by: Tim Deegan <tim@xxxxxxx>
---
 xen/include/asm-arm/arm32/spinlock.h |   25 ++++++++++++++-----------
 xen/include/asm-arm/arm64/spinlock.h |    3 ++-
 2 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/xen/include/asm-arm/arm32/spinlock.h 
b/xen/include/asm-arm/arm32/spinlock.h
index 4a11a97..ba11ad6 100644
--- a/xen/include/asm-arm/arm32/spinlock.h
+++ b/xen/include/asm-arm/arm32/spinlock.h
@@ -34,17 +34,20 @@ static always_inline void _raw_spin_unlock(raw_spinlock_t 
*lock)
 
 static always_inline int _raw_spin_trylock(raw_spinlock_t *lock)
 {
-    unsigned long tmp;
-
-    __asm__ __volatile__(
-"   ldrex   %0, [%1]\n"
-"   teq     %0, #0\n"
-"   strexeq %0, %2, [%1]"
-    : "=&r" (tmp)
-    : "r" (&lock->lock), "r" (1)
-    : "cc");
-
-    if (tmp == 0) {
+    unsigned long contended, res;
+
+    do {
+        __asm__ __volatile__(
+    "   ldrex   %0, [%2]\n"
+    "   teq     %0, #0\n"
+    "   strexeq %1, %3, [%2]\n"
+    "   movne   %1, #0\n"
+        : "=&r" (contended), "=r" (res)
+        : "r" (&lock->lock), "r" (1)
+        : "cc");
+    } while (res);
+
+    if (!contended) {
         smp_mb();
         return 1;
     } else {
diff --git a/xen/include/asm-arm/arm64/spinlock.h 
b/xen/include/asm-arm/arm64/spinlock.h
index 717f2fe..3a36cfd 100644
--- a/xen/include/asm-arm/arm64/spinlock.h
+++ b/xen/include/asm-arm/arm64/spinlock.h
@@ -40,9 +40,10 @@ static always_inline int _raw_spin_trylock(raw_spinlock_t 
*lock)
     unsigned int tmp;
 
     asm volatile(
-        "       ldaxr   %w0, %1\n"
+        "2:     ldaxr   %w0, %1\n"
         "       cbnz    %w0, 1f\n"
         "       stxr    %w0, %w2, %1\n"
+        "       cbnz    %w0, 2b\n"
         "1:\n"
         : "=&r" (tmp), "+Q" (lock->lock)
         : "r" (1)
--
generated by git-patchbot for /home/xen/git/xen.git#master

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