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fs: reimplement flushing

we only need to delay the rflush until the in flight
requests are done; do that.

--- a/dat.h

+++ b/dat.h

@@ -37,6 +37,7 @@

 	Nrefbuf	= 1024,			/* number of ref incs before syncing */

 	Nfidtab	= 1024,			/* number of fit hash entries */

+	Nflushtab = 1024,		/* flush table size */

 	Ndtab	= 1024,			/* number of dir tab entries */

 	Max9p	= 32*KiB,		/* biggest message size we're willing to negotiate */

 	Nsec	= 1000LL*1000*1000,	/* nanoseconds to the second */

@@ -377,7 +378,6 @@

 	Fcall;

 	Conn	*conn;

 	int	sz;	/* the size of the message buf */

-	Fmsg	*flush;	/* the flush message */

 	Amsg	*a;	/* admin messages */

 	uchar	buf[];

 };

@@ -508,8 +508,7 @@

 	usize	ccount;

 	usize	cmax;

-	Lock	mflushlk;

-	Fmsg	*mflush[Maxtag];

+	RWLock	flushq[Nflushtab];

 	Stats	stats;

 };

--- a/fs.c

+++ b/fs.c

@@ -194,7 +194,7 @@

 static void

 respond(Fmsg *m, Fcall *r)

 {

-	Fcall rf;

+	RWLock *lk;

 	uchar buf[Max9p+IOHDRSZ];

 	int w, n;

@@ -205,14 +205,11 @@

 	w = write(m->conn->wfd, buf, n);

 	if(w != n)

 		fshangup(m->conn, Eio);

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

-		lock(&fs->mflushlk);

-		fs->mflush[m->tag] = nil;

-		rf.type = Rflush;

-		if(m->flush != nil)

-			respond(m->flush, &rf);

-		unlock(&fs->mflushlk);

-	}

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

+	if(m->type == Tflush)

+		wunlock(lk);

+	else

+		runlock(lk);

 	free(m);

 }

@@ -660,7 +657,6 @@

 	m->conn = c;

 	m->sz = sz;

 	m->a = nil;

-	m->flush = nil;

 	PBIT32(m->buf, sz);

 	*pm = m;

 	return 0;

@@ -2040,20 +2036,10 @@

 void

 fsflush(Fmsg *m)

 {

-	Fmsg *o;

 	Fcall r;

-	lock(&fs->mflushlk);

-	o = fs->mflush[m->oldtag];

-	if(o != nil){

-		if(o->flush != nil)

-			free(o->flush);

-		o->flush = m;

-	}else{

-		r.type = Rflush;

-		respond(m, &r);

-	}

-	unlock(&fs->mflushlk);

+	r.type = Rflush;

+	respond(m, &r);

 }

 Conn *

@@ -2076,6 +2062,7 @@

 void

 runfs(int, void *pc)

 {

+	RWLock *lk;

 	Conn *c;

 	u32int h;

 	char err[128];

@@ -2100,9 +2087,11 @@

 		}

 		dprint("← %F\n", &m->Fcall);

-		lock(&fs->mflushlk);

-		fs->mflush[m->tag] = m;

-		unlock(&fs->mflushlk);

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

+		if(m->type == Tflush)

+			wlock(lk);

+		else

+			rlock(lk);

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

 		switch(m->type){