parisc: Improve LWS-CAS performance (take 2)

Commit Message

John David Anglin May 15, 2014, 12:33 p.m. UTC

On 13-May-14, at 11:31 PM, Carlos O'Donell wrote:

>> 2) Removes the lock code on UP systems where it is not needed, and
>
> This is wrong. The locking was originally only for SMP, but at one
> point it was discovered that you could COW during the CAS operation
> and if you didn't take the lock in UP, you could re-enter the CAS via
> another task and thus incorrectly carry out the CAS in two different
> threads and corrupt the state.

The attached file updates the change so that the lock code is no  
longer removed
on UP systems.  I added a comment to indicate that contention can  
occur even on
UP systems.

Lightly tested with a build of kyotocabinet on c3750.

Signed-off-by: John David Anglin <[e-mail]>
---

--
John David Anglin	[e-mail]

Patch

diff --git a/arch/parisc/kernel/syscall.S b/arch/parisc/kernel/syscall.S
index a63bb179..8387860 100644
--- a/arch/parisc/kernel/syscall.S
+++ b/arch/parisc/kernel/syscall.S
@@ -589,10 +589,13 @@  cas_nocontend:
 # endif
 /* ENABLE_LWS_DEBUG */
 
+	rsm	PSW_SM_I, %r0				/* Disable interrupts */
+	/* COW breaks can cause contention on UP systems */
 	LDCW	0(%sr2,%r20), %r28			/* Try to acquire the lock */
 	cmpb,<>,n	%r0, %r28, cas_action		/* Did we get it? */
 cas_wouldblock:
 	ldo	2(%r0), %r28				/* 2nd case */
+	ssm	PSW_SM_I, %r0
 	b	lws_exit				/* Contended... */
 	ldo	-EAGAIN(%r0), %r21			/* Spin in userspace */
 
@@ -619,15 +622,17 @@  cas_action:
 	stw	%r1, 4(%sr2,%r20)
 #endif
 	/* The load and store could fail */
-1:	ldw	0(%sr3,%r26), %r28
+1:	ldw,ma	0(%sr3,%r26), %r28
 	sub,<>	%r28, %r25, %r0
-2:	stw	%r24, 0(%sr3,%r26)
+2:	stw,ma	%r24, 0(%sr3,%r26)
 	/* Free lock */
-	stw	%r20, 0(%sr2,%r20)
+	stw,ma	%r20, 0(%sr2,%r20)
 #if ENABLE_LWS_DEBUG
 	/* Clear thread register indicator */
 	stw	%r0, 4(%sr2,%r20)
 #endif
+	/* Enable interrupts */
+	ssm	PSW_SM_I, %r0
 	/* Return to userspace, set no error */
 	b	lws_exit
 	copy	%r0, %r21
@@ -639,6 +644,7 @@  cas_action:
 #if ENABLE_LWS_DEBUG
 	stw	%r0, 4(%sr2,%r20)
 #endif
+	ssm	PSW_SM_I, %r0
 	b	lws_exit
 	ldo	-EFAULT(%r0),%r21	/* set errno */
 	nop