code: plan9front

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5e: approximate LL/SC with cas()

--- a/sys/src/cmd/5e/arm.c

+++ b/sys/src/cmd/5e/arm.c

@@ -120,10 +120,13 @@

 		*Rn = addr;

 }

+/* atomic compare and swap from libc */

+extern int cas(u32int *p, u32int old, u32int new);

+

 static void

 swap(u32int instr)

 {

-	u32int *Rm, *Rn, *Rd, *targ, addr, tmp;

+	u32int *Rm, *Rn, *Rd, *targ, addr, old, new;

 	Segment *seg;

 	Rm = P->R + (instr & 15);

@@ -134,19 +137,21 @@

 	addr = *Rn;

 	if((instr & fB) == 0)

 		addr = evenaddr(addr, 3);

-	clrex();

-	targ = (u32int *) vaddr(addr, 4, &seg);

-	lock(&seg->lock);

+	targ = (u32int *) vaddr(addr & ~3, 4, &seg);

+	do {

+		old = *targ;

+		new = *Rm;

+		if(instr & fB){

+			new &= 0xFF;

+			new <<= 8*(addr&3);

+			new |= old & ~(0xFF << 8*(addr&3));

+		}

+	} while(!cas(targ, old, new));

 	if(instr & fB) {

-		tmp = *(u8int*) targ;

-		*(u8int*) targ = *Rm;

-		*Rd = tmp;

-	} else {

-		tmp = *targ;

-		*targ = *Rm;

-		*Rd = tmp;

+		old >>= 8*(addr&3);

+		old &= 0xFF;

 	}

-	unlock(&seg->lock);

+	*Rd = old;

 	segunlock(seg);

 }

@@ -401,11 +406,10 @@

 		invalid(instr);

 	addr = evenaddr(*Rn, 3);

 	if(instr & fS) {

-		clrex();

 		targ = vaddr(addr, 4, &seg);

-		lock(&seg->lock);

-		P->excl = seg;

 		*Rd = *targ;

+		P->lladdr = addr;

+		P->llval = *Rd;

 		segunlock(seg);

 	} else {

 		Rm = P->R + (instr & 15);

@@ -412,15 +416,14 @@

 		if(Rm == P->R + 15)

 			invalid(instr);

 		targ = vaddr(addr, 4, &seg);

-		if(canlock(&seg->lock)) {

-			unlock(&seg->lock);

-			*Rd = 1;

-		} else if(P->excl != seg) {

-			*Rd = 1;

-		} else {

-			*targ = *Rm;

-			*Rd = 0;

-		}

+

+		/*

+		 * this is not quite correct as we will succeed even

+		 * if the value was modified and then restored to its

+		 * original value but good enougth approximation for

+		 * libc's _tas(), _cas() and _xinc()/_xdec().

+		 */

+		*Rd = addr != P->lladdr || !cas(targ, P->llval, *Rm);

 		segunlock(seg);

 		clrex();

 	}

@@ -429,12 +432,8 @@

 void

 clrex(void)

 {

-	Segment *seg;

-

-	seg = P->excl;

-	P->excl = nil;

-	if(seg != nil)

-		unlock(&seg->lock);

+	P->lladdr = 0;

+	P->llval = 0;

 }

 static void

--- a/sys/src/cmd/5e/dat.h

+++ b/sys/src/cmd/5e/dat.h

@@ -31,7 +31,10 @@

 	Ref *path;		/* Ref + string data */

 	Segment *S[SEGNUM];	/* memory */

-	Segment *excl;		/* recently acquired exclusive access */

+

+	u32int lladdr;		/* LL/SC emulation */

+	u32int llval;

+

 	u32int R[16];		/* general purpose registers / PC (R15) */

 	u32int CPSR;		/* status register */

@@ -66,7 +69,6 @@

 	Ref;

 	int flags;

 	RWLock rw; /* lock for SEGFLLOCK segments */

-	Lock lock; /* atomic accesses */

 	u32int start, size;

 	void *data;

 	Ref *dref;