kernel/spinlock.c: add default arch_*_relax definitions for GENERIC_LOCKBREAK

When running with GENERIC_LOCKBREAK=y, the locking implementations emit
calls to arch_{read,write,spin}_relax when spinning on a contended lock
in order to allow architectures to favour the CPU owning the lock if
possible.

In reality, everybody apart from PowerPC and S390 just does cpu_relax()
here, so make that the default behaviour and allow it to be overridden
if required.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/spinlock.c b/kernel/spinlock.c
index 5cdd8065a3ce32f6cecadd5a99a59a8ebc937613..4b082b5cac9eedfd7c31daef7e8dd02974eedbed 100644 (file)
--- a/kernel/spinlock.c
+++ b/kernel/spinlock.c
@@ -34,6 +34,20 @@
 #else
 #define raw_read_can_lock(l)   read_can_lock(l)
 #define raw_write_can_lock(l)  write_can_lock(l)
+
+/*
+ * Some architectures can relax in favour of the CPU owning the lock.
+ */
+#ifndef arch_read_relax
+# define arch_read_relax(l)    cpu_relax()
+#endif
+#ifndef arch_write_relax
+# define arch_write_relax(l)   cpu_relax()
+#endif
+#ifndef arch_spin_relax
+# define arch_spin_relax(l)    cpu_relax()
+#endif
+
 /*
  * We build the __lock_function inlines here. They are too large for
  * inlining all over the place, but here is only one user per function