parisc: Rewrite light-weight syscall and futex code

The parisc architecture lacks general hardware support for compare and swap.
Particularly for userspace, it is difficult to implement software atomic
support. Page faults in critical regions can cause processes to sleep and
block the forward progress of other processes.  Thus, it is essential that
page faults be disabled in critical regions. For performance reasons, we
also need to disable external interrupts in critical regions.

In order to do this, we need a mechanism to trigger COW breaks outside the
critical region. Fortunately, parisc has the "stbys,e" instruction. When
the leftmost byte of a word is addressed, this instruction triggers all
the exceptions of a normal store but it does not write to memory. Thus,
we can use it to trigger COW breaks outside the critical region without
modifying the data that is to be updated atomically.

COW breaks occur randomly.  So even if we have priviously executed a "stbys,e"
instruction, we still need to disable pagefaults around the critical region.
If a fault occurs in the critical region, we return -EAGAIN. I had to add
a wrapper around _arch_futex_atomic_op_inuser() as I found in testing that
returning -EAGAIN caused problems for some processes even though it is
listed as a possible return value.

The patch implements the above. The code no longer attempts to sleep with
interrupts disabled and I haven't seen any stalls with the change.

I have attempted to merge common code and streamline the fast path.  In the
futex code, we only compute the spinlock address once.

I eliminated some debug code in the original CAS routine that just made the
flow more complicated.

I don't clip the arguments when called from wide mode. As a result, the LWS
routines should work when called from 64-bit processes.

I defined TASK_PAGEFAULT_DISABLED offset for use in the lws_pagefault_disable
and lws_pagefault_enable macros.

Since we now disable interrupts on the gateway page where necessary, it
might be possible to allow processes to be scheduled when they are on the
gateway page.

Change has been tested on c8000 and rp3440. It improves glibc build and test
time by about 10%.

In v2, I removed the lws_atomic_xchg and and lws_atomic_store calls. I
also removed the bug fixes that were not directly related to this patch.

In v3, I removed the code to force interruptions from
arch_futex_atomic_op_inuser(). It is always called with page faults
disabled, so this code had no effect.

In v4, I fixed a typo in depi_safe line.

In v5, I moved the code to disable/enable page faults inside the spinlocks.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>

3 files changed, 231 insertions, 212 deletions

diff --git a/arch/parisc/include/asm/futex.h b/arch/parisc/include/asm/futex.h
index 9cd4dd6e63ad..b5835325d44b 100644
--- a/arch/parisc/include/asm/futex.h
+++ b/arch/parisc/include/asm/futex.h
@@ -8,39 +8,47 @@
 #include <asm/errno.h>
 
 /* The following has to match the LWS code in syscall.S.  We have
-   sixteen four-word locks. */
+ * 256 four-word locks. We use bits 20-27 of the futex virtual
+ * address for the hash index.
+ */
+
+static inline unsigned long _futex_hash_index(unsigned long ua)
+{
+	return (ua >> 2) & 0x3fc;
+}
 
 static inline void
-_futex_spin_lock(u32 __user *uaddr)
+_futex_spin_lock_irqsave(arch_spinlock_t *s, unsigned long *flags)
 {
-	extern u32 lws_lock_start[];
-	long index = ((long)uaddr & 0x7f8) >> 1;
-	arch_spinlock_t *s = (arch_spinlock_t *)&lws_lock_start[index];
-	preempt_disable();
+	local_irq_save(*flags);
 	arch_spin_lock(s);
 }
 
 static inline void
-_futex_spin_unlock(u32 __user *uaddr)
+_futex_spin_unlock_irqrestore(arch_spinlock_t *s, unsigned long *flags)
 {
-	extern u32 lws_lock_start[];
-	long index = ((long)uaddr & 0x7f8) >> 1;
-	arch_spinlock_t *s = (arch_spinlock_t *)&lws_lock_start[index];
 	arch_spin_unlock(s);
-	preempt_enable();
+	local_irq_restore(*flags);
 }
 
 static inline int
 arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
 {
+	extern u32 lws_lock_start[];
+	unsigned long ua = (unsigned long)uaddr;
+	arch_spinlock_t *s;
+	unsigned long flags;
 	int oldval, ret;
 	u32 tmp;
 
-	ret = -EFAULT;
+	s = (arch_spinlock_t *)&lws_lock_start[_futex_hash_index(ua)];
+	_futex_spin_lock_irqsave(s, &flags);
 
-	_futex_spin_lock(uaddr);
-	if (unlikely(get_user(oldval, uaddr) != 0))
+	/* Return -EFAULT if we encounter a page fault or COW break */
+	if (unlikely(get_user(oldval, uaddr) != 0)) {
+		ret = -EFAULT;
 		goto out_pagefault_enable;
+	}
 
 	ret = 0;
 	tmp = oldval;
@@ -63,13 +71,14 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
 		break;
 	default:
 		ret = -ENOSYS;
+		goto out_pagefault_enable;
 	}
 
-	if (ret == 0 && unlikely(put_user(tmp, uaddr) != 0))
+	if (unlikely(put_user(tmp, uaddr) != 0))
 		ret = -EFAULT;
 
 out_pagefault_enable:
-	_futex_spin_unlock(uaddr);
+	_futex_spin_unlock_irqrestore(s, &flags);
 
 	if (!ret)
 		*oval = oldval;
@@ -81,7 +90,11 @@ static inline int
 futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 			      u32 oldval, u32 newval)
 {
+	extern u32 lws_lock_start[];
+	unsigned long ua = (unsigned long)uaddr;
+	arch_spinlock_t *s;
 	u32 val;
+	unsigned long flags;
 
 	/* futex.c wants to do a cmpxchg_inatomic on kernel NULL, which is
 	 * our gateway page, and causes no end of trouble...
@@ -94,23 +107,25 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 
 	/* HPPA has no cmpxchg in hardware and therefore the
 	 * best we can do here is use an array of locks. The
-	 * lock selected is based on a hash of the userspace
-	 * address. This should scale to a couple of CPUs.
+	 * lock selected is based on a hash of the virtual
+	 * address of the futex. This should scale to a couple
+	 * of CPUs.
 	 */
 
-	_futex_spin_lock(uaddr);
+	s = (arch_spinlock_t *)&lws_lock_start[_futex_hash_index(ua)];
+	_futex_spin_lock_irqsave(s, &flags);
 	if (unlikely(get_user(val, uaddr) != 0)) {
-		_futex_spin_unlock(uaddr);
+		_futex_spin_unlock_irqrestore(s, &flags);
 		return -EFAULT;
 	}
 
 	if (val == oldval && unlikely(put_user(newval, uaddr) != 0)) {
-		_futex_spin_unlock(uaddr);
+		_futex_spin_unlock_irqrestore(s, &flags);
 		return -EFAULT;
 	}
 
 	*uval = val;
-	_futex_spin_unlock(uaddr);
+	_futex_spin_unlock_irqrestore(s, &flags);
 
 	return 0;
 }
diff --git a/arch/parisc/kernel/asm-offsets.c b/arch/parisc/kernel/asm-offsets.c
index 55c1c5189c6a..396aa3b47712 100644
--- a/arch/parisc/kernel/asm-offsets.c
+++ b/arch/parisc/kernel/asm-offsets.c
@@ -37,6 +37,7 @@ int main(void)
 {
 	DEFINE(TASK_TI_FLAGS, offsetof(struct task_struct, thread_info.flags));
 	DEFINE(TASK_STACK, offsetof(struct task_struct, stack));
+	DEFINE(TASK_PAGEFAULT_DISABLED, offsetof(struct task_struct, pagefault_disabled));
 	BLANK();
 	DEFINE(TASK_REGS, offsetof(struct task_struct, thread.regs));
 	DEFINE(TASK_PT_PSW, offsetof(struct task_struct, thread.regs.gr[ 0]));
diff --git a/arch/parisc/kernel/syscall.S b/arch/parisc/kernel/syscall.S
index 65c88ca7a7ac..027a43e1930d 100644
--- a/arch/parisc/kernel/syscall.S
+++ b/arch/parisc/kernel/syscall.S
@@ -50,6 +50,22 @@ registers).
 
 	.level          PA_ASM_LEVEL
 
+	.macro	lws_pagefault_disable reg1,reg2
+	mfctl	%cr30, \reg2
+	ldo	TASK_PAGEFAULT_DISABLED(\reg2), \reg2
+	ldw	0(%sr2,\reg2), \reg1
+	ldo	1(\reg1), \reg1
+	stw	\reg1, 0(%sr2,\reg2)
+	.endm
+
+	.macro	lws_pagefault_enable reg1,reg2
+	mfctl	%cr30, \reg2
+	ldo	TASK_PAGEFAULT_DISABLED(\reg2), \reg2
+	ldw	0(%sr2,\reg2), \reg1
+	ldo	-1(\reg1), \reg1
+	stw	\reg1, 0(%sr2,\reg2)
+	.endm
+
 	.text
 
 	.import syscall_exit,code
@@ -490,8 +506,34 @@ lws_start:
 	/* Jump to lws, lws table pointers already relocated */
 	be,n	0(%sr2,%r21)
 
+lws_exit_noerror:
+	lws_pagefault_enable	%r1,%r21
+	stw,ma	%r20, 0(%sr2,%r20)
+	ssm	PSW_SM_I, %r0
+	b	lws_exit
+	copy	%r0, %r21
+
+lws_wouldblock:
+	ssm	PSW_SM_I, %r0
+	ldo	2(%r0), %r28
+	b	lws_exit
+	ldo	-EAGAIN(%r0), %r21
+
+lws_pagefault:
+	lws_pagefault_enable	%r1,%r21
+	stw,ma	%r20, 0(%sr2,%r20)
+	ssm	PSW_SM_I, %r0
+	ldo	3(%r0),%r28
+	b	lws_exit
+	ldo	-EAGAIN(%r0),%r21
+
+lws_fault:
+	ldo	1(%r0),%r28
+	b	lws_exit
+	ldo	-EFAULT(%r0),%r21
+
 lws_exit_nosys:
-	ldo	-ENOSYS(%r0),%r21		   /* set errno */
+	ldo	-ENOSYS(%r0),%r21
 	/* Fall through: Return to userspace */
 
 lws_exit:
@@ -518,27 +560,19 @@ lws_exit:
 		%r28 - Return prev through this register.
 		%r21 - Kernel error code
 
-		If debugging is DISabled:
-
-		%r21 has the following meanings:
-
+		%r21 returns the following error codes:
 		EAGAIN - CAS is busy, ldcw failed, try again.
 		EFAULT - Read or write failed.		
 
-		If debugging is enabled:
-
-		EDEADLOCK - CAS called recursively.
-		EAGAIN && r28 == 1 - CAS is busy. Lock contended.
-		EAGAIN && r28 == 2 - CAS is busy. ldcw failed.
-		EFAULT - Read or write failed.
+		If EAGAIN is returned, %r28 indicates the busy reason:
+		r28 == 1 - CAS is busy. lock contended.
+		r28 == 2 - CAS is busy. ldcw failed.
+		r28 == 3 - CAS is busy. page fault.
 
 		Scratch: r20, r28, r1
 
 	****************************************************/
 
-	/* Do not enable LWS debugging */
-#define ENABLE_LWS_DEBUG 0 
-
 	/* ELF64 Process entry path */
 lws_compare_and_swap64:
 #ifdef CONFIG_64BIT
@@ -551,59 +585,45 @@ lws_compare_and_swap64:
 	b,n	lws_exit_nosys
 #endif
 
-	/* ELF32 Process entry path */
+	/* ELF32/ELF64 Process entry path */
 lws_compare_and_swap32:
 #ifdef CONFIG_64BIT
-	/* Clip all the input registers */
+	/* Wide mode user process? */
+	bb,<,n  %sp, 31, lws_compare_and_swap
+
+	/* Clip all the input registers for 32-bit processes */
 	depdi	0, 31, 32, %r26
 	depdi	0, 31, 32, %r25
 	depdi	0, 31, 32, %r24
 #endif
 
 lws_compare_and_swap:
-	/* Load start of lock table */
-	ldil	L%lws_lock_start, %r20
-	ldo	R%lws_lock_start(%r20), %r28
+	/* Trigger memory reference interruptions without writing to memory */
+1:	ldw	0(%r26), %r28
+2:	stbys,e	%r0, 0(%r26)
+
+	/* Calculate 8-bit hash index from virtual address */
+	extru_safe	%r26, 27, 8, %r20
 
-	/* Extract eight bits from r26 and hash lock (Bits 3-11) */
-	extru_safe  %r26, 28, 8, %r20
+	/* Load start of lock table */
+	ldil	L%lws_lock_start, %r28
+	ldo	R%lws_lock_start(%r28), %r28
 
-	/* Find lock to use, the hash is either one of 0 to
-	   15, multiplied by 16 (keep it 16-byte aligned)
+	/* Find lock to use, the hash index is one of 0 to
+	   255, multiplied by 16 (keep it 16-byte aligned)
 	   and add to the lock table offset. */
 	shlw	%r20, 4, %r20
 	add	%r20, %r28, %r20
 
-# if ENABLE_LWS_DEBUG
-	/*	
-		DEBUG, check for deadlock! 
-		If the thread register values are the same
-		then we were the one that locked it last and
-		this is a recurisve call that will deadlock.
-		We *must* giveup this call and fail.
-	*/
-	ldw	4(%sr2,%r20), %r28			/* Load thread register */
-	/* WARNING: If cr27 cycles to the same value we have problems */
-	mfctl	%cr27, %r21				/* Get current thread register */
-	cmpb,<>,n	%r21, %r28, cas_lock		/* Called recursive? */
-	b	lws_exit				/* Return error! */
-	ldo	-EDEADLOCK(%r0), %r21
-cas_lock:
-	cmpb,=,n	%r0, %r28, cas_nocontend	/* Is nobody using it? */
-	ldo	1(%r0), %r28				/* 1st case */
-	b	lws_exit				/* Contended... */
-	ldo	-EAGAIN(%r0), %r21			/* Spin in userspace */
-cas_nocontend:
-# endif
-/* ENABLE_LWS_DEBUG */
-
-	/* 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 */
-	b	lws_exit				/* Contended... */
-	ldo	-EAGAIN(%r0), %r21			/* Spin in userspace */
+	rsm	PSW_SM_I, %r0				/* Disable interrupts */
+
+	/* Try to acquire the lock */
+	LDCW	0(%sr2,%r20), %r28
+	comclr,<>	%r0, %r28, %r0
+	b,n	lws_wouldblock
+
+	/* Disable page faults to prevent sleeping in critical region */
+	lws_pagefault_disable	%r21,%r28
 
 	/*
 		prev = *addr;
@@ -613,59 +633,35 @@ cas_wouldblock:
 	*/
 
 	/* NOTES:
-		This all works becuse intr_do_signal
+		This all works because intr_do_signal
 		and schedule both check the return iasq
 		and see that we are on the kernel page
 		so this process is never scheduled off
 		or is ever sent any signal of any sort,
-		thus it is wholly atomic from usrspaces
+		thus it is wholly atomic from usrspace's
 		perspective
 	*/
-cas_action:
-#if defined CONFIG_SMP && ENABLE_LWS_DEBUG
-	/* DEBUG */
-	mfctl	%cr27, %r1
-	stw	%r1, 4(%sr2,%r20)
-#endif
 	/* The load and store could fail */
-1:	ldw	0(%r26), %r28
+3:	ldw	0(%r26), %r28
 	sub,<>	%r28, %r25, %r0
-2:	stw	%r24, 0(%r26)
-	/* Free lock */
-	stw,ma	%r20, 0(%sr2,%r20)
-#if ENABLE_LWS_DEBUG
-	/* Clear thread register indicator */
-	stw	%r0, 4(%sr2,%r20)
-#endif
-	/* Return to userspace, set no error */
-	b	lws_exit
-	copy	%r0, %r21
+4:	stw	%r24, 0(%r26)
+	b,n	lws_exit_noerror
 
-3:		
-	/* Error occurred on load or store */
-	/* Free lock */
-	stw,ma	%r20, 0(%sr2,%r20)
-#if ENABLE_LWS_DEBUG
-	stw	%r0, 4(%sr2,%r20)
-#endif
-	b	lws_exit
-	ldo	-EFAULT(%r0),%r21	/* set errno */
-	nop
-	nop
-	nop
-	nop
+	/* A fault occurred on load or stbys,e store */
+5:	b,n	lws_fault
+	ASM_EXCEPTIONTABLE_ENTRY(1b-linux_gateway_page, 5b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(2b-linux_gateway_page, 5b-linux_gateway_page)
 
-	/* Two exception table entries, one for the load,
-	   the other for the store. Either return -EFAULT.
-	   Each of the entries must be relocated. */
-	ASM_EXCEPTIONTABLE_ENTRY(1b-linux_gateway_page, 3b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(2b-linux_gateway_page, 3b-linux_gateway_page)
+	/* A page fault occurred in critical region */
+6:	b,n	lws_pagefault
+	ASM_EXCEPTIONTABLE_ENTRY(3b-linux_gateway_page, 6b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(4b-linux_gateway_page, 6b-linux_gateway_page)
 
 
 	/***************************************************
 		New CAS implementation which uses pointers and variable size
 		information. The value pointed by old and new MUST NOT change
-		while performing CAS. The lock only protect the value at %r26.
+		while performing CAS. The lock only protects the value at %r26.
 
 		%r26 - Address to examine
 		%r25 - Pointer to the value to check (old)
@@ -674,25 +670,32 @@ cas_action:
 		%r28 - Return non-zero on failure
 		%r21 - Kernel error code
 
-		%r21 has the following meanings:
-
+		%r21 returns the following error codes:
 		EAGAIN - CAS is busy, ldcw failed, try again.
 		EFAULT - Read or write failed.
 
+		If EAGAIN is returned, %r28 indicates the busy reason:
+		r28 == 1 - CAS is busy. lock contended.
+		r28 == 2 - CAS is busy. ldcw failed.
+		r28 == 3 - CAS is busy. page fault.
+
 		Scratch: r20, r22, r28, r29, r1, fr4 (32bit for 64bit CAS only)
 
 	****************************************************/
 
-	/* ELF32 Process entry path */
 lws_compare_and_swap_2:
 #ifdef CONFIG_64BIT
-	/* Clip the input registers. We don't need to clip %r23 as we
-	   only use it for word operations */
+	/* Wide mode user process? */
+	bb,<,n	%sp, 31, cas2_begin
+
+	/* Clip the input registers for 32-bit processes. We don't
+	   need to clip %r23 as we only use it for word operations */
 	depdi	0, 31, 32, %r26
 	depdi	0, 31, 32, %r25
 	depdi	0, 31, 32, %r24
 #endif
 
+cas2_begin:
 	/* Check the validity of the size pointer */
 	subi,>>= 3, %r23, %r0
 	b,n	lws_exit_nosys
@@ -703,69 +706,77 @@ lws_compare_and_swap_2:
 	blr	%r29, %r0
 	nop
 
-	/* 8bit load */
-4:	ldb	0(%r25), %r25
+	/* 8-bit load */
+1:	ldb	0(%r25), %r25
 	b	cas2_lock_start
-5:	ldb	0(%r24), %r24
+2:	ldb	0(%r24), %r24
 	nop
 	nop
 	nop
 	nop
 	nop
 
-	/* 16bit load */
-6:	ldh	0(%r25), %r25
+	/* 16-bit load */
+3:	ldh	0(%r25), %r25
 	b	cas2_lock_start
-7:	ldh	0(%r24), %r24
+4:	ldh	0(%r24), %r24
 	nop
 	nop
 	nop
 	nop
 	nop
 
-	/* 32bit load */
-8:	ldw	0(%r25), %r25
+	/* 32-bit load */
+5:	ldw	0(%r25), %r25
 	b	cas2_lock_start
-9:	ldw	0(%r24), %r24
+6:	ldw	0(%r24), %r24
 	nop
 	nop
 	nop
 	nop
 	nop
 
-	/* 64bit load */
+	/* 64-bit load */
 #ifdef CONFIG_64BIT
-10:	ldd	0(%r25), %r25
-11:	ldd	0(%r24), %r24
+7:	ldd	0(%r25), %r25
+8:	ldd	0(%r24), %r24
 #else
 	/* Load old value into r22/r23 - high/low */
-10:	ldw	0(%r25), %r22
-11:	ldw	4(%r25), %r23
+7:	ldw	0(%r25), %r22
+8:	ldw	4(%r25), %r23
 	/* Load new value into fr4 for atomic store later */
-12:	flddx	0(%r24), %fr4
+9:	flddx	0(%r24), %fr4
 #endif
 
 cas2_lock_start:
-	/* Load start of lock table */
-	ldil	L%lws_lock_start, %r20
-	ldo	R%lws_lock_start(%r20), %r28
+	/* Trigger memory reference interruptions without writing to memory */
+	copy	%r26, %r28
+	depi_safe	0, 31, 2, %r28
+10:	ldw	0(%r28), %r1
+11:	stbys,e	%r0, 0(%r28)
 
-	/* Extract eight bits from r26 and hash lock (Bits 3-11) */
-	extru_safe  %r26, 28, 8, %r20
+	/* Calculate 8-bit hash index from virtual address */
+	extru_safe	%r26, 27, 8, %r20
 
-	/* Find lock to use, the hash is either one of 0 to
-	   15, multiplied by 16 (keep it 16-byte aligned)
+	/* Load start of lock table */
+	ldil	L%lws_lock_start, %r28
+	ldo	R%lws_lock_start(%r28), %r28
+
+	/* Find lock to use, the hash index is one of 0 to
+	   255, multiplied by 16 (keep it 16-byte aligned)
 	   and add to the lock table offset. */
 	shlw	%r20, 4, %r20
 	add	%r20, %r28, %r20
 
-	/* COW breaks can cause contention on UP systems */
-	LDCW	0(%sr2,%r20), %r28		/* Try to acquire the lock */
-	cmpb,<>,n	%r0, %r28, cas2_action	/* Did we get it? */
-cas2_wouldblock:
-	ldo	2(%r0), %r28			/* 2nd case */
-	b	lws_exit			/* Contended... */
-	ldo	-EAGAIN(%r0), %r21		/* Spin in userspace */
+	rsm	PSW_SM_I, %r0			/* Disable interrupts */
+
+	/* Try to acquire the lock */
+	LDCW	0(%sr2,%r20), %r28
+	comclr,<>	%r0, %r28, %r0
+	b,n	lws_wouldblock
+
+	/* Disable page faults to prevent sleeping in critical region */
+	lws_pagefault_disable	%r21,%r28
 
 	/*
 		prev = *addr;
@@ -775,110 +786,102 @@ cas2_wouldblock:
 	*/
 
 	/* NOTES:
-		This all works becuse intr_do_signal
+		This all works because intr_do_signal
 		and schedule both check the return iasq
 		and see that we are on the kernel page
 		so this process is never scheduled off
 		or is ever sent any signal of any sort,
-		thus it is wholly atomic from usrspaces
+		thus it is wholly atomic from usrspace's
 		perspective
 	*/
-cas2_action:
+
 	/* Jump to the correct function */
 	blr	%r29, %r0
 	/* Set %r28 as non-zero for now */
 	ldo	1(%r0),%r28
 
-	/* 8bit CAS */
-13:	ldb	0(%r26), %r29
+	/* 8-bit CAS */
+12:	ldb	0(%r26), %r29
 	sub,=	%r29, %r25, %r0
-	b,n	cas2_end
-14:	stb	%r24, 0(%r26)
-	b	cas2_end
+	b,n	lws_exit_noerror
+13:	stb	%r24, 0(%r26)
+	b	lws_exit_noerror
 	copy	%r0, %r28
 	nop
 	nop
 
-	/* 16bit CAS */
-15:	ldh	0(%r26), %r29
+	/* 16-bit CAS */
+14:	ldh	0(%r26), %r29
 	sub,=	%r29, %r25, %r0
-	b,n	cas2_end
-16:	sth	%r24, 0(%r26)
-	b	cas2_end
+	b,n	lws_exit_noerror
+15:	sth	%r24, 0(%r26)
+	b	lws_exit_noerror
 	copy	%r0, %r28
 	nop
 	nop
 
-	/* 32bit CAS */
-17:	ldw	0(%r26), %r29
+	/* 32-bit CAS */
+16:	ldw	0(%r26), %r29
 	sub,=	%r29, %r25, %r0
-	b,n	cas2_end
-18:	stw	%r24, 0(%r26)
-	b	cas2_end
+	b,n	lws_exit_noerror
+17:	stw	%r24, 0(%r26)
+	b	lws_exit_noerror
 	copy	%r0, %r28
 	nop
 	nop
 
-	/* 64bit CAS */
+	/* 64-bit CAS */
 #ifdef CONFIG_64BIT
-19:	ldd	0(%r26), %r29
+18:	ldd	0(%r26), %r29
 	sub,*=	%r29, %r25, %r0
-	b,n	cas2_end
-20:	std	%r24, 0(%r26)
+	b,n	lws_exit_noerror
+19:	std	%r24, 0(%r26)
 	copy	%r0, %r28
 #else
 	/* Compare first word */
-19:	ldw	0(%r26), %r29
+18:	ldw	0(%r26), %r29
 	sub,=	%r29, %r22, %r0
-	b,n	cas2_end
+	b,n	lws_exit_noerror
 	/* Compare second word */
-20:	ldw	4(%r26), %r29
+19:	ldw	4(%r26), %r29
 	sub,=	%r29, %r23, %r0
-	b,n	cas2_end
+	b,n	lws_exit_noerror
 	/* Perform the store */
-21:	fstdx	%fr4, 0(%r26)
+20:	fstdx	%fr4, 0(%r26)
 	copy	%r0, %r28
 #endif
+	b	lws_exit_noerror
+	copy	%r0, %r28
 
-cas2_end:
-	/* Free lock */
-	stw,ma	%r20, 0(%sr2,%r20)
-	/* Return to userspace, set no error */
-	b	lws_exit
-	copy	%r0, %r21
-
-22:
-	/* Error occurred on load or store */
-	/* Free lock */
-	stw,ma	%r20, 0(%sr2,%r20)
-	ldo	1(%r0),%r28
-	b	lws_exit
-	ldo	-EFAULT(%r0),%r21	/* set errno */
-	nop
-	nop
-	nop
+	/* A fault occurred on load or stbys,e store */
+30:	b,n	lws_fault
+	ASM_EXCEPTIONTABLE_ENTRY(1b-linux_gateway_page, 30b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(2b-linux_gateway_page, 30b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(3b-linux_gateway_page, 30b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(4b-linux_gateway_page, 30b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(5b-linux_gateway_page, 30b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(6b-linux_gateway_page, 30b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(7b-linux_gateway_page, 30b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(8b-linux_gateway_page, 30b-linux_gateway_page)
+#ifndef CONFIG_64BIT
+	ASM_EXCEPTIONTABLE_ENTRY(9b-linux_gateway_page, 30b-linux_gateway_page)
+#endif
 
-	/* Exception table entries, for the load and store, return EFAULT.
-	   Each of the entries must be relocated. */
-	ASM_EXCEPTIONTABLE_ENTRY(4b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(5b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(6b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(7b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(8b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(9b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(10b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(11b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(13b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(14b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(15b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(16b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(17b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(18b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(19b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(20b-linux_gateway_page, 22b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(10b-linux_gateway_page, 30b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(11b-linux_gateway_page, 30b-linux_gateway_page)
+
+	/* A page fault occurred in critical region */
+31:	b,n	lws_pagefault
+	ASM_EXCEPTIONTABLE_ENTRY(12b-linux_gateway_page, 31b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(13b-linux_gateway_page, 31b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(14b-linux_gateway_page, 31b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(15b-linux_gateway_page, 31b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(16b-linux_gateway_page, 31b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(17b-linux_gateway_page, 31b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(18b-linux_gateway_page, 31b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(19b-linux_gateway_page, 31b-linux_gateway_page)
 #ifndef CONFIG_64BIT
-	ASM_EXCEPTIONTABLE_ENTRY(12b-linux_gateway_page, 22b-linux_gateway_page)
-	ASM_EXCEPTIONTABLE_ENTRY(21b-linux_gateway_page, 22b-linux_gateway_page)
+	ASM_EXCEPTIONTABLE_ENTRY(20b-linux_gateway_page, 31b-linux_gateway_page)
 #endif
 
 	/* Make sure nothing else is placed on this page */
@@ -899,7 +902,7 @@ ENTRY(end_linux_gateway_page)
 ENTRY(lws_table)
 	LWS_ENTRY(compare_and_swap32)		/* 0 - ELF32 Atomic 32bit CAS */
 	LWS_ENTRY(compare_and_swap64)		/* 1 - ELF64 Atomic 32bit CAS */
-	LWS_ENTRY(compare_and_swap_2)		/* 2 - ELF32 Atomic 64bit CAS */
+	LWS_ENTRY(compare_and_swap_2)		/* 2 - Atomic 64bit CAS */
 END(lws_table)
 	/* End of lws table */