shithub: gefs

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ref: ccbf2ae6831a064e4115aa0ad2181b304dfaeebc
parent: 8585477a0d205a33c9d89a69f73c7ebf4727cd72
author: Ori Bernstein <ori@eigenstate.org>
date: Mon Nov 13 22:46:30 EST 2023 

blk: preallocate log blocks to avoid deadlocks

if we have no blocks left in to pluck, we can end
up in a situation where the free ops that we have
queued can end up stuck, so we never end up with
new blocks available for the plucking; this change
allocates the blocks at FS startup, so we can never
have the sync proc waiting for blocks to get freed


--- a/blk.c
+++ b/blk.c
@@ -234,18 +234,15 @@
 }
 
 static Blk*
-mklogblk(vlong o)
+mklogblk(Arena *a, vlong o)
 {
 	Blk *lb;
 
-	if((lb = cachepluck()) == nil)
-		return nil;
+	lb = a->logbuf[a->lbidx++ % nelem(a->logbuf)];
 	initblk(lb, o, -1, Tlog);
 	finalize(lb);
-	if(syncblk(lb) == -1){
-		dropblk(lb);
+	if(syncblk(lb) == -1)
 		return nil;
-	}
 	return lb;
 }
 
@@ -289,7 +286,7 @@
 	if(lb == nil || lb->logsz >= Logspc - Logslop){
 		if((o = blkalloc_lk(a, 1)) == -1)
 			return -1;
-		if((nl = mklogblk(o)) == nil)
+		if((nl = mklogblk(a, o)) == nil)
 			return -1;
 		p = lb->data + lb->logsz;
 		PACK64(p, o|LogAlloc1);
@@ -301,7 +298,6 @@
 			fs->broken = 1;
 			return -1;
 		}
-		dropblk(lb);
 		a->logtl = nl;
 		a->nlog++;
 		lb = nl;
--- a/dat.h
+++ b/dat.h
@@ -517,6 +517,7 @@
 	long	epoch;
 	long	lepoch[32];
 	Bfree	*limbo[3];
+	long	nlimbo;
 
 	Syncq	syncq[32];
 
@@ -565,9 +566,11 @@
 	Avltree *free;
 	Blk	**queue;
 	int	nqueue;
-	Blk	*hd;			/* arena header */
-	Blk	*tl;			/* arena footer */
-	Blk	**q;			/* write queue */
+	int	lbidx;
+	Blk	*logbuf[2];	/* preallocated log pages */
+	Blk	*hd;		/* arena header */
+	Blk	*tl;		/* arena footer */
+	Blk	**q;		/* write queue */
 	vlong	nq;
 	vlong	size;
 	vlong	used;
@@ -598,6 +601,8 @@
 	Key;
 	Xdir;
 	Dent	*next;
+	QLock	trunclk;
+	int	truncating;
 	vlong	up;
 	long	ref;
 	char	gone;
--- a/load.c
+++ b/load.c
@@ -102,6 +102,8 @@
 	}
 	for(i = 0; i < fs->narena; i++){
 		a = &fs->arenas[i];
+		a->logbuf[0] = cachepluck();
+		a->logbuf[1] = cachepluck();
 		if(loadlog(a, a->loghd) == -1)
 			sysfatal("load log %B: %r", a->loghd);
 	}