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kernel: fix deadlock between sysexec() and killbig() (thanks ori)

During exec, we used to hold up->seglock while the stack
is being demand paged in.

In a low-memory situation, this leads to deadlock because
killbig() is calling procpagecount() for each process
to determine is physical memory usage. This function
acqires p->seglock. So killbig() can be stuck once it
hits a process that is starving on memory during faulting
in its stack.

This change makes sysexec release the seglock during its
its stack preparation phase, and re-aquire it once we
enter the "committ" phase (freeing our old stack).

--- a/sys/src/9/port/sysproc.c

+++ b/sys/src/9/port/sysproc.c

@@ -443,15 +443,9 @@

 	 */

 	qlock(&up->seglock);

 	if(waserror()){

-		s = up->seg[ESEG];

-		if(s != nil){

-			up->seg[ESEG] = nil;

-			putseg(s);

-		}

 		qunlock(&up->seglock);

 		nexterror();

 	}

-

 	s = up->seg[SSEG];

 	do {

 		tstk = s->base;

@@ -459,7 +453,18 @@

 			error(Enovmem);

 	} while((s = isoverlap(tstk-USTKSIZE, USTKSIZE)) != nil);

 	up->seg[ESEG] = newseg(SG_STACK | SG_NOEXEC, tstk-USTKSIZE, USTKSIZE/BY2PG);

+	qunlock(&up->seglock);

+	if(waserror()){

+		qlock(&up->seglock);

+		s = up->seg[ESEG];

+		if(s != nil){

+			up->seg[ESEG] = nil;

+			putseg(s);

+		}

+		nexterror();

+	}

+

 	/*

 	 * Args: pass 2: assemble; the pages will be faulted in

 	 */

@@ -520,6 +525,9 @@

 	 * Free old memory.

 	 * Special segments are maintained across exec

 	 */

+	poperror();

+	qlock(&up->seglock);

+

 	for(i = SSEG; i <= BSEG; i++) {

 		s = up->seg[i];

 		if(s != nil) {

@@ -576,10 +584,14 @@

 	 * Move the stack

 	 */

 	s = up->seg[ESEG];

+	if(s == nil)

+		error(Egreg);

 	up->seg[ESEG] = nil;

+	qlock(s);

 	s->base = USTKTOP-USTKSIZE;

 	s->top = USTKTOP;

 	relocateseg(s, USTKTOP-tstk);

+	qunlock(s);

 	up->seg[SSEG] = s;

 	qunlock(&up->seglock);

 	poperror();	/* seglock */

@@ -894,8 +906,8 @@

 		qunlock(s);

 		error(Ebadarg);

 	}

-	up->seg[i] = nil;

 	qunlock(s);

+	up->seg[i] = nil;

 	putseg(s);

 	qunlock(&up->seglock);

 	poperror();

-- 

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