shithub: gefs

Download patch

ref: 76d83e6d63310062f9e7f01a007085c7622d2ca6
parent: ccbf2ae6831a064e4115aa0ad2181b304dfaeebc
author: Ori Bernstein <ori@eigenstate.org>
date: Tue Nov 14 00:47:46 EST 2023 

gefs: fix block exhaustion deadlocks

This commit bundles 2 related changes together to
prevent block exhaustion. First, we split out the
lock semantics for truncation, which has two benefits;
first, it allows readers to read truncated files, but
more importantly, we can now block on getting the
dent outside of an epoch, allowing the sweeper to
clean blocks and reclaim them.

Second, we now stall in epochclean if our limbo
list is getting long; this means that with small
caches, we make sure we have enough for the next
epoch before we start it.


--- a/blk.c
+++ b/blk.c
@@ -820,10 +820,26 @@
 }
 
 void
+limbo(Bfree *f)
+{
+	Bfree *p;
+	ulong ge;
+
+	while(1){
+		ge = agetl(&fs->epoch);
+		p = agetp(&fs->limbo[ge]);
+		f->next = p;
+		if(acasp(&fs->limbo[ge], p, f)){
+			aincl(&fs->nlimbo, 1);
+			break;
+		}
+	}
+}
+
+void
 freeblk(Tree *t, Blk *b, Bptr bp)
 {
 	Bfree *f;
-	ulong ge;
 
 	if(t == &fs->snap || (t != nil && bp.gen <= t->gen)){
 		killblk(t, bp);
@@ -840,10 +856,7 @@
 		b->freed = getcallerpc(&t);
 		f->b = holdblk(b);
 	}
-
-	ge = agetl(&fs->epoch);
-	f->next = fs->limbo[ge];
-	fs->limbo[ge] = f;
+	limbo(f);
 }
 
 void
@@ -867,17 +880,28 @@
 void
 epochclean(void)
 {
-	ulong e, ge;
+	ulong c, e, ge, delay;
 	Bfree *p, *n;
 	Arena *a;
 	Qent qe;
 	int i;
 
+	delay = 0;
+Again:
+	c = agetl(&fs->nlimbo);
 	ge = agetl(&fs->epoch);
 	for(i = 0; i < fs->nworker; i++){
 		e = agetl(&fs->lepoch[i]);
-		if((e & Eactive) && e != (ge | Eactive))
-			return;
+		if((e & Eactive) && e != (ge | Eactive)){
+			if(c < fs->cmax/4)
+				return;
+			if(delay < 100)
+				delay++;
+			else
+				fprint(2, "stalled epoch %lx [worker %d]\n", e, i);
+			sleep(delay);
+			goto Again;
+		}
 	}
 	p = asetp(&fs->limbo[(ge+1)%3], nil);
 	asetl(&fs->epoch, (ge+1)%3);
@@ -903,6 +927,7 @@
 		default:
 			abort();
 		}
+		aincl(&fs->nlimbo, -1);
 		free(p);
 	}
 }
--- a/dat.h
+++ b/dat.h
@@ -602,7 +602,6 @@
 	Xdir;
 	Dent	*next;
 	QLock	trunclk;
-	int	truncating;
 	vlong	up;
 	long	ref;
 	char	gone;
--- a/fns.h
+++ b/fns.h
@@ -52,7 +52,7 @@
 void	epochstart(int);
 void	epochend(int);
 void	epochclean(void);
-void	freesync(void);
+void	limbo(Bfree*);
 void	freeblk(Tree*, Blk*, Bptr);
 int	dlappend(Dlist *dl, Bptr);
 int	killblk(Tree*, Bptr);
--- a/fs.c
+++ b/fs.c
@@ -259,6 +259,18 @@
 	return btupsert(mnt->root, m, nm);
 }
 
+static void
+truncwait(Dent *de, int id)
+{
+	if(canqlock(&de->trunclk))
+		return;
+	epochend(id);
+	qunlock(&fs->mutlk);
+	qlock(&de->trunclk);
+	qlock(&fs->mutlk);
+	epochstart(id);
+}
+
 static int
 readb(Fid *f, char *d, vlong o, vlong n, vlong sz)
 {
@@ -1164,7 +1176,7 @@
 }
 
 static void
-fswstat(Fmsg *m, Amsg **ao)
+fswstat(Fmsg *m, int id, Amsg **ao)
 {
 	char rnbuf[Kvmax], opbuf[Kvmax], upbuf[Upksz];
 	char *p, *e, strs[65535];
@@ -1188,6 +1200,7 @@
 		return;
 	}
 	de = f->dent;
+	truncwait(de, id);
 	wlock(de);
 	if(de->gone){
 		rerror(m, Ephase);
@@ -1393,6 +1406,7 @@
 	}
 	assert(nm <= nelem(mb));
 	if((e = upsert(f->mnt, mb, nm)) != nil){
+		qunlock(&de->trunclk);
 		wunlock(de);
 		rerror(m, e);
 		goto Out;
@@ -1407,7 +1421,8 @@
 
 Out:
 	if(!truncate)
-		wunlock(de);
+		qunlock(&de->trunclk);
+	wunlock(de);
 	putfid(f);
 }
 
@@ -1576,7 +1591,7 @@
 }
 
 static void
-fsremove(Fmsg *m, Amsg **ao)
+fsremove(Fmsg *m, int id, Amsg **ao)
 {
 	char upbuf[Upksz];
 	Fcall r;
@@ -1590,6 +1605,7 @@
 	}
 	clunkfid(m->conn, f);
 
+	truncwait(f->dent, id);
 	wlock(f->dent);
 	if(f->dent->gone){
 		e = Ephase;
@@ -1626,6 +1642,7 @@
 		(*ao)->dent = nil;
 	}
 	f->dent->gone = 1;
+	qunlock(&f->dent->trunclk);
 	wunlock(f->dent);
 
 	r.type = Rremove;
@@ -1634,6 +1651,7 @@
 	return;
 
 Error:
+	qunlock(&f->dent->trunclk);
 	wunlock(f->dent);
 	rerror(m, e);
 	putfid(f);
@@ -1640,7 +1658,7 @@
 }
 
 static void
-fsopen(Fmsg *m, Amsg **ao)
+fsopen(Fmsg *m, int id, Amsg **ao)
 {
 	char *p, *e, buf[Kvmax];
 	int mbits;
@@ -1706,6 +1724,7 @@
 //		unlock(&fs->root.lk);
 //	}
 	if(m->mode & OTRUNC){
+		truncwait(f->dent, id);
 		wlock(f->dent);
 		f->dent->muid = f->uid;
 		f->dent->qid.vers++;
@@ -1732,12 +1751,13 @@
 		(*ao)->length = f->dent->length;
 		(*ao)->dent = nil;
 		if((e = upsert(f->mnt, &mb, 1)) != nil){
-Error:
+Error:			qunlock(&f->dent->trunclk);
 			wunlock(f->dent);
 			rerror(m, e);
 			putfid(f);
 			return;
 		}
+		qunlock(&f->dent->trunclk);
 		wunlock(f->dent);
 	}
 	unlock(f);
@@ -1938,7 +1958,7 @@
 }
 
 static void
-fswrite(Fmsg *m)
+fswrite(Fmsg *m, int id)
 {
 	char sbuf[Wstatmax], kbuf[Max9p/Blksz+2][Offksz], vbuf[Max9p/Blksz+2][Ptrsz];
 	Bptr bp[Max9p/Blksz + 2];
@@ -1959,9 +1979,11 @@
 		putfid(f);
 		return;
 	}
+	truncwait(f->dent, id);
 	wlock(f->dent);
 	if(f->dent->gone){
 		rerror(m, Ephase);
+		qunlock(&f->dent->trunclk);
 		wunlock(f->dent);
 		putfid(f);
 		return;
@@ -1972,6 +1994,7 @@
 			rerror(m, e);
 		else
 			respond(m, &r);
+		qunlock(&f->dent->trunclk);
 		wunlock(f->dent);
 		putfid(f);
 		return;
@@ -1993,6 +2016,7 @@
 		if(n == -1){
 			for(j = 0; j < i; j++)
 				freeblk(t, nil, bp[i]);
+			qunlock(&f->dent->trunclk);
 			wunlock(f->dent);
 			fprint(2, "%r");
 			putfid(f);
@@ -2029,11 +2053,13 @@
 	kv[i].nv = p - sbuf;
 	if((e = upsert(f->mnt, kv, i+1)) != nil){
 		rerror(m, e);
+		qunlock(&f->dent->trunclk);
 		wunlock(f->dent);
 		putfid(f);
 		abort();
 		return;
 	}
+	qunlock(&f->dent->trunclk);
 	wunlock(f->dent);
 
 	r.type = Rwrite;
@@ -2164,12 +2190,12 @@
 		qlock(&fs->mutlk);
 		epochstart(id);
 		switch(m->type){
-		case Tcreate:	fscreate(m);	break;
-		case Twrite:	fswrite(m);	break;
-		case Twstat:	fswstat(m, &a);	break;
-		case Tremove:	fsremove(m,&a);	break;
-		case Topen:	fsopen(m, &a);	break;
-		default:	abort();	break;
+		case Tcreate:	fscreate(m);		break;
+		case Twrite:	fswrite(m, id);		break;
+		case Twstat:	fswstat(m, id, &a);	break;
+		case Tremove:	fsremove(m, id, &a);	break;
+		case Topen:	fsopen(m, id, &a);	break;
+		default:	abort();		break;
 		}
  		epochend(id);
  		qunlock(&fs->mutlk);
@@ -2189,11 +2215,11 @@
 		m = chrecv(ch);
 		epochstart(id);
 		switch(m->type){
-		case Tattach:	fsattach(m);	break;
-		case Twalk:	fswalk(m);	break;
-		case Tread:	fsread(m);	break;
-		case Tstat:	fsstat(m);	break;
-		case Topen:	fsopen(m, nil);	break;
+		case Tattach:	fsattach(m);		break;
+		case Twalk:	fswalk(m);		break;
+		case Tread:	fsread(m);		break;
+		case Tstat:	fsstat(m);		break;
+		case Topen:	fsopen(m, id, nil);	break;
 		}
 		epochend(id);
 	}
@@ -2298,7 +2324,7 @@
 				epochclean();
 			}
 			if(am->dent != nil){
-				wunlock(am->dent);
+				qunlock(&am->dent->trunclk);
 				clunkdent(am->dent);
 			}
 			clunkmount(am->mnt);
--- a/snap.c
+++ b/snap.c
@@ -513,7 +513,6 @@
 closesnap(Tree *t)
 {
 	Bfree *f;
-	ulong ge;
 
 	if(t == nil || adec(&t->memref) != 0)
 		return;
@@ -521,9 +520,7 @@
 		abort();
 	f->op = DFtree;
 	f->t = t;
-	ge = agetl(&fs->epoch);
-	f->next = fs->limbo[ge];
-	fs->limbo[ge] = f;
+	limbo(f);
 }
 
 char*