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sdvirtio: be more carefull at wakeup

wakeup cannot access r->sleep once we set r->done because the
sleeper might just return at this point making r invalid. make
a copy of the sleep rendez pointer before setting r->done.

--- a/sys/src/9/pc/sdvirtio.c

+++ b/sys/src/9/pc/sdvirtio.c

@@ -218,6 +218,7 @@

 {

 	int id, free, m;

 	struct Rock *r;

+	Rendez *z;

 	Vqueue *q;

 	Vdev *vd;

@@ -231,8 +232,10 @@

 			id = q->usedent[q->lastused++ & m].id;

 			if(r = q->rock[id]){

 				q->rock[id] = nil;

-				r->done = 1;

-				wakeup(r->sleep);

+				z = r->sleep;

+				r->done = 1;	/* hands off */

+				if(z != nil)

+					wakeup(z);

 			}

 			do {

 				free = id;

-- 

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