parisc: Do not use an ordered store in pa_tlb_lock()

No need to use an ordered store in pa_tlb_lock() and update the comment
regarng usage of the sid register to unlocak a spinlock in
tlb_unlock0().

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: <stable@vger.kernel.org> # v5.0+

arch/parisc/kernel/entry.S

@@ -455,15 +455,20 @@
 	LDREG		0(\ptp),\pte
 	bb,<,n		\pte,_PAGE_PRESENT_BIT,3f
 	b		\fault
-	stw,ma		\spc,0(\tmp)
+	stw		\spc,0(\tmp)
 99:	ALTERNATIVE(98b, 99b, ALT_COND_NO_SMP, INSN_NOP)
 #endif
 2:	LDREG		0(\ptp),\pte
 	bb,>=,n		\pte,_PAGE_PRESENT_BIT,\fault
 3:
 	.endm
 
-	/* Release pa_tlb_lock lock without reloading lock address. */
+	/* Release pa_tlb_lock lock without reloading lock address.
+	   Note that the values in the register spc are limited to
+	   NR_SPACE_IDS (262144). Thus, the stw instruction always
+	   stores a nonzero value even when register spc is 64 bits.
+	   We use an ordered store to ensure all prior accesses are
+	   performed prior to releasing the lock. */
 	.macro		tlb_unlock0	spc,tmp
 #ifdef CONFIG_SMP
 98:	or,COND(=)	%r0,\spc,%r0