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gefs: avoid holding the wrlock when syncing the file system

we only need to enforce ordering or syncedness to disk when
writing out the blocks during sync, we don't need to prevent
mutation on the tree once the arenas and superblock are
serialized.

--- a/sys/src/cmd/gefs/blk.c

+++ b/sys/src/cmd/gefs/blk.c

@@ -153,8 +153,10 @@

 	lo = 0;

 	hi = fs->narena;

-	if(b == 0)

+	if(b == fs->sb0->bp.addr)

 		return &fs->arenas[0];

+	if(b == fs->sb1->bp.addr)

+		return &fs->arenas[hi-1];

 	while(1){

 		mid = (hi + lo)/2;

 		a = &fs->arenas[mid];

@@ -1000,7 +1002,7 @@

 	int i;

 	if(qe.op == Qfree || qe.op == Qwrite)

-		assert(qe.bp.addr != 0 && (qe.bp.addr & (Blksz-1)) == 0);

+		assert((qe.bp.addr & (Blksz-1)) == 0);

 	else if(qe.op == Qfence)

 		assert(fs->syncing > 0);

 	else

--- a/sys/src/cmd/gefs/fs.c

+++ b/sys/src/cmd/gefs/fs.c

@@ -38,11 +38,18 @@

 static void

 wrbarrier(void)

 {

+	tracev("barrier", fs->qgen);

+	aincv(&fs->qgen, 1);

+}

+

+static void

+wrwait(void)

+{

 	Qent qe;

 	int i;

-	

+

+	tracev("wrwait", fs->qgen);

 	aincv(&fs->qgen, 1);

-	tracev("barrier", fs->qgen);

 	fs->syncing = fs->nsyncers;

 	for(i = 0; i < fs->nsyncers; i++){

 		qe.op = Qfence;

@@ -130,6 +137,8 @@

 	packsb(fs->sb1->buf, Blksz, fs);

 	finalize(fs->sb0);

 	finalize(fs->sb1);

+	setflag(fs->sb0, Bdirty);

+	setflag(fs->sb1, Bdirty);

 	fs->snap.dirty = 0;

 	qunlock(&fs->mutlk);

@@ -150,10 +159,10 @@

 	 * get synced after so that we can use them next

 	 * time around.

          */

-	qlock(&fs->mutlk);

 	tracem("supers");

-	syncblk(fs->sb0);

-	syncblk(fs->sb1);

+	enqueue(fs->sb0);

+	enqueue(fs->sb1);

+	wrbarrier();

 	/*

 	 * pass 3: sync block footers; if we crash here,

@@ -165,11 +174,13 @@

 		enqueue(fs->arenas[i].h1);

 	/*

-	 * Pass 4: clean up the old snap tree's deadlist

+	 * Pass 4: clean up the old snap tree's deadlist.

+	 * we need to wait for all the new data to hit disk

+	 * before we can free anything, otherwise it gets

+	 * clobbered.

 	 */

 	tracem("snapdl");

-	wrbarrier();

-	qunlock(&fs->mutlk);

+	wrwait();

 	freedl(&dl, 1);

 	qunlock(&fs->synclk);

 	tracem("synced");

-- 

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