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snap: free deadlist blocks directly

unbounded amounts of deferred reclamation are bad,
but because of the way that these trees are deadlisted,
all the blocks are already guaranteed to be unreachable,
so we don't need to defer the reclamation at all.

--- a/snap.c

+++ b/snap.c

@@ -151,7 +151,9 @@

 freedl(Dlist *dl, int docontents)

 {

 	char buf[Kvmax];

-	Bptr bp, fb;

+	Arena *a;

+	Qent qe;

+	Bptr bp;

 	Msg m;

 	Blk *b;

 	char *p;

@@ -164,19 +166,33 @@

 	}

 	while(bp.addr != -1){

 		b = getblk(bp, 0);

+		/*

+		 * Because these deadlists are dead-dead at this point,

+		 * they'll never be read from again; we can avoid worrying

+		 * about deferred reclamation, and queue them up to be freed

+		 * directly, which means we don't need to worry about watiing

+		 * for a quiescent state, and the associated out-of-block

+		 * deadlocks that come with it.

+		 */

 		if(docontents){

 			for(p = b->data; p != b->data+b->logsz; p += 8){

-				fb.addr = UNPACK64(p);

-				fb.hash = -1;

-				fb.gen = -1;

-				traceb("dlclear", fb);

-				freeblk(nil, nil, fb);

+				qe.op = Qfree;

+				qe.bp.addr = UNPACK64(p);

+				qe.bp.hash = -1;

+				qe.bp.gen = -1;

+				qe.b = nil;

+				a = getarena(qe.bp.addr);

+				qput(a->sync, qe);

+				traceb("dlclear", qe.bp);

 			}

 		}

 		bp = b->logp;

-		traceb("dlfreeb", b->bp);

-		freeblk(nil, b, b->bp);

-		dropblk(b);

+		qe.op = Qfree;

+		qe.bp = b->bp;

+		qe.b = b;

+		a = getarena(qe.bp.addr);

+		qput(a->sync, qe);

+		traceb("dlfreeb", qe.bp);

 	}

 }