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fs: correct flush behavior

we want to wlock the *old* tag in flush

--- a/fs.c

+++ b/fs.c

@@ -211,11 +211,13 @@

 	qunlock(&m->conn->wrlk);

 	if(w != n)

 		fshangup(m->conn, Eio);

-	lk = &fs->flushq[ihash(m->tag) % Nflushtab];

-	if(m->type == Tflush)

+	if(m->type == Tflush){

+		lk = &fs->flushq[ihash(m->oldtag) % Nflushtab];

 		wunlock(lk);

-	else

+	}else{

+		lk = &fs->flushq[ihash(m->tag) % Nflushtab];

 		runlock(lk);

+	}

 	free(m);

 }

@@ -2069,12 +2071,12 @@

 runfs(int, void *pc)

 {

 	char err[128];

-	long old;

-	u32int h;

 	RWLock *lk;

 	Conn *c;

 	Fcall r;

 	Fmsg *m;

+	u32int h;

+	long old;

 	c = pc;

 	while(1){

@@ -2098,11 +2100,13 @@

 			fprint(2, "dup incoming tag: %d [type=%ld]\n", m->tag, old);

 			abort();

 		}

-		lk = &fs->flushq[ihash(m->tag) % Nflushtab];

-		if(m->type == Tflush)

+		if(m->type == Tflush){

+			lk = &fs->flushq[ihash(m->oldtag) % Nflushtab];

 			wlock(lk);

-		else

+		}else{

+			lk = &fs->flushq[ihash(m->tag) % Nflushtab];

 			rlock(lk);

+		}

 		h = ihash(m->fid) % fs->nreaders;

 		switch(m->type){