dtracy: avoid dmachlock() race

between being commited to a machno and having acquired the lock, the
scheduler could come in an schedule us on a different processor. the
solution is to have dtmachlock() take a special -1 argument to mean
"current mach" and return the actual mach number after the lock has
been acquired and interrupts being disabled.

1 files changed, 9 insertions, 1 deletions

diff --git a/sys/src/9/port/devdtracy.c b/sys/src/9/port/devdtracy.c
index 018bad624..19a321cc5 100644
--- a/sys/src/9/port/devdtracy.c
+++ b/sys/src/9/port/devdtracy.c
@@ -512,10 +512,18 @@ dtrealloc(void *v, ulong n)
 	return v;
 }
 
-void
+int
 dtmachlock(int i)
 {
+	while(i < 0) {
+		i = dtmachlock(m->machno);
+		if(i == m->machno)
+			return i;
+		dtmachunlock(i);
+		i = -1;
+	}
 	ilock(&machlocks[i]);
+	return i;
 }
 
 void