[4.1] 32af38d Add a final backstop, so we absolutely 100% certainly do not try to delete a objhead while it still has a waiting list, by forcing the last ref holder to rush the WL.

[Lasse Karstensen]

commit 32af38d43c9cefa10ef24775ef999fbd61fd49ed
Author: Poul-Henning Kamp <phk at FreeBSD.org>
Date:   Thu Jan 7 00:03:35 2016 +0000

    Add a final backstop, so we absolutely 100% certainly do not
    try to delete a objhead while it still has a waiting list,
    by forcing the last ref holder to rush the WL.
    
    Since the hasher owns the refcounts on objhead, we cannot just
    mingle req and objcore refcounts.
    
    Fortunately we don't need to add another refcounter, because all
    we really care about is the wl being empty when we drop the last
    ref.
    
    The wl/hsh_rush() mechanism will work differently with different
    thread-scheduling schenarios, and I cannot definitively rule out
    that we can drop the last ref on an oh, while there are still req's
    on the waiting list.
    
    Given that, and the existence proof in ticket #1823's race, this
    might have been the indicated memory-trampler.
    
    Conflicts:
    	bin/varnishd/cache/cache_hash.c

diff --git a/bin/varnishd/cache/cache_hash.c b/bin/varnishd/cache/cache_hash.c
index 0bb827d..18b9628 100644
--- a/bin/varnishd/cache/cache_hash.c
+++ b/bin/varnishd/cache/cache_hash.c
@@ -820,10 +820,25 @@ HSH_DerefObjHead(struct worker *wrk, struct objhead **poh)
 		oh->refcnt--;
 		Lck_Unlock(&oh->mtx);
 		return(1);
-	} else if (oh->waitinglist != NULL) {
+	}
+
+	/*
+	 * Make absolutely certain that we do not let the final ref
+	 * disappear until the waitinglist is empty.
+	 * This is necessary because the req's on the waiting list do
+	 * not hold any ref on the objhead of their own, and we cannot
+	 * just make the hold the same ref's as objcore, that would
+	 * confuse hashers.
+	 */
+	while (oh->waitinglist != NULL) {
 		Lck_Lock(&oh->mtx);
+		assert(oh->refcnt > 0);
+		r = oh->refcnt;
 		hsh_rush(wrk, oh);
 		Lck_Unlock(&oh->mtx);
+		if (r > 1)
+			break;
+		usleep(100000);
 	}
 
 	assert(oh->refcnt > 0);
diff --git a/bin/varnishd/hash/hash_critbit.c b/bin/varnishd/hash/hash_critbit.c
index c4239b6..944985d 100644
--- a/bin/varnishd/hash/hash_critbit.c
+++ b/bin/varnishd/hash/hash_critbit.c
@@ -394,9 +394,7 @@ hcb_start(void)
 static int __match_proto__(hash_deref_f)
 hcb_deref(struct objhead *oh)
 {
-	int r;
 
-	r = 1;
 	CHECK_OBJ_NOTNULL(oh, OBJHEAD_MAGIC);
 	Lck_Lock(&oh->mtx);
 	assert(oh->refcnt > 0);
@@ -413,7 +411,7 @@ hcb_deref(struct objhead *oh)
 #ifdef PHK
 	fprintf(stderr, "hcb_defef %d %d <%s>\n", __LINE__, r, oh->hash);
 #endif
-	return (r);
+	return (1);
 }
 
 static struct objhead * __match_proto__(hash_lookup_f)