MySQL Bug#72754 - Set thread priority in InnoDB mutex spinloop

On POWER, you can set thread priority rather than Intel relax cpu instruction

So the correct way to write a spin loop for POWER is to lower the thread

priority while you spin and raise it up afterwards.

We add the asm volatile("":::"memory"); bit of asm to make GCC not optimize

the spin loop down to a nop. This is a GCC barrier rather than a CPU memory

barrier - and is arguably needed for x86 as well

Index: mysql-5.6.17/storage/innobase/sync/sync0rw.cc

===================================================================

--- mysql-5.6.17.orig/storage/innobase/sync/sync0rw.cc	2014-05-26 12:16:25.622071001 +1000

+++ mysql-5.6.17/storage/innobase/sync/sync0rw.cc	2014-05-26 17:37:14.702071000 +1000

@@ -382,6 +382,7 @@

 	/* Spin waiting for the writer field to become free */

 	os_rmb;

+	asm volatile ("or 1,1,1");

 	while (i < SYNC_SPIN_ROUNDS && lock->lock_word <= 0) {

 		if (srv_spin_wait_delay) {

 			ut_delay(ut_rnd_interval(0, srv_spin_wait_delay));

@@ -393,7 +394,9 @@

 #else

 		i++;

 #endif /* HAVE_MEMORY_BARRIER */

+		asm volatile ("":::"memory");

 	}

+	asm volatile ("or 2,2,2");

 	if (i >= SYNC_SPIN_ROUNDS) {

 		os_thread_yield();

Index: mysql-5.6.17/storage/innobase/sync/sync0sync.cc

===================================================================

--- mysql-5.6.17.orig/storage/innobase/sync/sync0sync.cc	2014-05-26 12:16:25.622071001 +1000

+++ mysql-5.6.17/storage/innobase/sync/sync0sync.cc	2014-05-26 17:37:52.152071000 +1000

@@ -501,6 +501,7 @@

 spin_loop:

 	os_rmb;

+	asm volatile("or 1,1,1");

 	while (mutex_get_lock_word(mutex) != 0 && i < SYNC_SPIN_ROUNDS) {

 		if (srv_spin_wait_delay) {

 			ut_delay(ut_rnd_interval(0, srv_spin_wait_delay));

@@ -512,7 +513,9 @@

 #else

 		i++;

 #endif /* HAVE_MEMORY_BARRIER */

+		asm volatile("":::"memory");

 	}

+	asm volatile ("or 2,2,2;");

 	if (i >= SYNC_SPIN_ROUNDS) {

 		os_thread_yield();