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authorAnton Arapov <anton@redhat.com>2012-04-16 10:05:28 +0200
committerAnton Arapov <anton@redhat.com>2012-04-16 10:05:28 +0200
commitb4b6116a13633898cf868f2f103c96a90c4c20f8 (patch)
tree93d1b7e2cfcdf473d8d4ff3ad141fa864f8491f6 /arch/arm/include/asm/mutex.h
parentedd4be777c953e5faafc80d091d3084b4343f5d3 (diff)
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fedora kernel: d9aad82f3319f3cfd1aebc01234254ef0c37ad84v3.3.2-1
Signed-off-by: Anton Arapov <anton@redhat.com>
Diffstat (limited to 'arch/arm/include/asm/mutex.h')
-rw-r--r--arch/arm/include/asm/mutex.h127
1 files changed, 127 insertions, 0 deletions
diff --git a/arch/arm/include/asm/mutex.h b/arch/arm/include/asm/mutex.h
new file mode 100644
index 00000000000..93226cf23ae
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+++ b/arch/arm/include/asm/mutex.h
@@ -0,0 +1,127 @@
+/*
+ * arch/arm/include/asm/mutex.h
+ *
+ * ARM optimized mutex locking primitives
+ *
+ * Please look into asm-generic/mutex-xchg.h for a formal definition.
+ */
+#ifndef _ASM_MUTEX_H
+#define _ASM_MUTEX_H
+
+#if __LINUX_ARM_ARCH__ < 6
+/* On pre-ARMv6 hardware the swp based implementation is the most efficient. */
+# include <asm-generic/mutex-xchg.h>
+#else
+
+/*
+ * Attempting to lock a mutex on ARMv6+ can be done with a bastardized
+ * atomic decrement (it is not a reliable atomic decrement but it satisfies
+ * the defined semantics for our purpose, while being smaller and faster
+ * than a real atomic decrement or atomic swap. The idea is to attempt
+ * decrementing the lock value only once. If once decremented it isn't zero,
+ * or if its store-back fails due to a dispute on the exclusive store, we
+ * simply bail out immediately through the slow path where the lock will be
+ * reattempted until it succeeds.
+ */
+static inline void
+__mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
+{
+ int __ex_flag, __res;
+
+ __asm__ (
+
+ "ldrex %0, [%2] \n\t"
+ "sub %0, %0, #1 \n\t"
+ "strex %1, %0, [%2] "
+
+ : "=&r" (__res), "=&r" (__ex_flag)
+ : "r" (&(count)->counter)
+ : "cc","memory" );
+
+ __res |= __ex_flag;
+ if (unlikely(__res != 0))
+ fail_fn(count);
+}
+
+static inline int
+__mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
+{
+ int __ex_flag, __res;
+
+ __asm__ (
+
+ "ldrex %0, [%2] \n\t"
+ "sub %0, %0, #1 \n\t"
+ "strex %1, %0, [%2] "
+
+ : "=&r" (__res), "=&r" (__ex_flag)
+ : "r" (&(count)->counter)
+ : "cc","memory" );
+
+ __res |= __ex_flag;
+ if (unlikely(__res != 0))
+ __res = fail_fn(count);
+ return __res;
+}
+
+/*
+ * Same trick is used for the unlock fast path. However the original value,
+ * rather than the result, is used to test for success in order to have
+ * better generated assembly.
+ */
+static inline void
+__mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
+{
+ int __ex_flag, __res, __orig;
+
+ __asm__ (
+
+ "ldrex %0, [%3] \n\t"
+ "add %1, %0, #1 \n\t"
+ "strex %2, %1, [%3] "
+
+ : "=&r" (__orig), "=&r" (__res), "=&r" (__ex_flag)
+ : "r" (&(count)->counter)
+ : "cc","memory" );
+
+ __orig |= __ex_flag;
+ if (unlikely(__orig != 0))
+ fail_fn(count);
+}
+
+/*
+ * If the unlock was done on a contended lock, or if the unlock simply fails
+ * then the mutex remains locked.
+ */
+#define __mutex_slowpath_needs_to_unlock() 1
+
+/*
+ * For __mutex_fastpath_trylock we use another construct which could be
+ * described as a "single value cmpxchg".
+ *
+ * This provides the needed trylock semantics like cmpxchg would, but it is
+ * lighter and less generic than a true cmpxchg implementation.
+ */
+static inline int
+__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
+{
+ int __ex_flag, __res, __orig;
+
+ __asm__ (
+
+ "1: ldrex %0, [%3] \n\t"
+ "subs %1, %0, #1 \n\t"
+ "strexeq %2, %1, [%3] \n\t"
+ "movlt %0, #0 \n\t"
+ "cmpeq %2, #0 \n\t"
+ "bgt 1b "
+
+ : "=&r" (__orig), "=&r" (__res), "=&r" (__ex_flag)
+ : "r" (&count->counter)
+ : "cc", "memory" );
+
+ return __orig;
+}
+
+#endif
+#endif