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From: John David Anglin <dave.anglin@bell.net>
To: linux-parisc <linux-parisc@vger.kernel.org>
Cc: Helge Deller <deller@gmx.de>,
	James Bottomley <James.Bottomley@HansenPartnership.com>
Subject: [PATCH] parisc: Use ordered store to release TLB lock
Date: Tue, 21 Jul 2020 07:57:01 -0400
Message-ID: <81ff8f16-2ff5-195a-b6d5-d7e3c970d097@bell.net> (raw)

The LDCW barriers in the tlb_lock and tlb_unlock0 macros can be removed.

In tlb_lock, the barrier isn't needed because there are no prior accesses to be concerned about.

In tlb_unlock0, we replace the LDCW barrier with an ordered store.  This ensures that all prior
accesses are performed before the store is performed.

Signed-off-by: Dave Anglin <dave.anglin@bell.net>
---

diff --git a/arch/parisc/kernel/entry.S b/arch/parisc/kernel/entry.S
index 4b484ec7c7da..60b966b29d6c 100644
--- a/arch/parisc/kernel/entry.S
+++ b/arch/parisc/kernel/entry.S
@@ -454,7 +454,6 @@
 	nop
 	LDREG		0(\ptp),\pte
 	bb,<,n		\pte,_PAGE_PRESENT_BIT,3f
-	LDCW		0(\tmp),\tmp1
 	b		\fault
 	stw		\spc,0(\tmp)
 99:	ALTERNATIVE(98b, 99b, ALT_COND_NO_SMP, INSN_NOP)
@@ -464,13 +463,16 @@
 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,tmp1
 #ifdef CONFIG_SMP
 98:	or,COND(=)	%r0,\spc,%r0
-	LDCW		0(\tmp),\tmp1
-	or,COND(=)	%r0,\spc,%r0
-	stw		\spc,0(\tmp)
+	stw,ma		\spc,0(\tmp)
 99:	ALTERNATIVE(98b, 99b, ALT_COND_NO_SMP, INSN_NOP)
 #endif
 	.endm

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