ARM: 7749/1: spinlock: retry trylock operation if strex fails on free lock

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.

Reported-by: Tony Thompson <anthony.thompson@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

diff --git a/arch/arm/include/asm/spinlock.h b/arch/arm/include/asm/spinlock.h
index 6220e9fdf4c7d0917fe50dc54b23f70502ea43a5..f8b8965666e9b14842786742f48ccdef49000802 100644 (file)
--- a/arch/arm/include/asm/spinlock.h
+++ b/arch/arm/include/asm/spinlock.h
@@ -97,19 +97,22 @@ static inline void arch_spin_lock(arch_spinlock_t *lock)
 
 static inline int arch_spin_trylock(arch_spinlock_t *lock)
 {
-       unsigned long tmp;
+       unsigned long contended, res;
        u32 slock;
 
-       __asm__ __volatile__(
-"      ldrex   %0, [%2]\n"
-"      subs    %1, %0, %0, ror #16\n"
-"      addeq   %0, %0, %3\n"
-"      strexeq %1, %0, [%2]"
-       : "=&r" (slock), "=&r" (tmp)
-       : "r" (&lock->slock), "I" (1 << TICKET_SHIFT)
-       : "cc");
-
-       if (tmp == 0) {
+       do {
+               __asm__ __volatile__(
+               "       ldrex   %0, [%3]\n"
+               "       mov     %2, #0\n"
+               "       subs    %1, %0, %0, ror #16\n"
+               "       addeq   %0, %0, %4\n"
+               "       strexeq %2, %0, [%3]"
+               : "=&r" (slock), "=&r" (contended), "=r" (res)
+               : "r" (&lock->slock), "I" (1 << TICKET_SHIFT)
+               : "cc");
+       } while (res);
+
+       if (!contended) {
                smp_mb();
                return 1;
        } else {