parisc: Define mb() and add memory barriers to assembler unlock sequences

For years I thought all parisc machines executed loads and stores in
order. However, Jeff Law recently indicated on gcc-patches that this is
not correct. There are various degrees of out-of-order execution all the
way back to the PA7xxx processor series (hit-under-miss). The PA8xxx
series has full out-of-order execution for both integer operations, and
loads and stores.

This is described in the following article:
http://web.archive.org/web/20040214092531/http://www.cpus.hp.com/technical_references/advperf.shtml

For this reason, we need to define mb() and to insert a memory barrier
before the store unlocking spinlocks. This ensures that all memory
accesses are complete prior to unlocking. The ldcw instruction performs
the same function on entry.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Cc: stable@vger.kernel.org # 4.0+
Signed-off-by: Helge Deller <deller@gmx.de>

arch/parisc/kernel/entry.S

@@ -481,6 +481,8 @@
 	/* Release pa_tlb_lock lock without reloading lock address. */
 	.macro		tlb_unlock0	spc,tmp
 #ifdef CONFIG_SMP
+	or,COND(=)	%r0,\spc,%r0
+	sync
 	or,COND(=)	%r0,\spc,%r0
 	stw             \spc,0(\tmp)
 #endif