stvfe_dskoc_fco: Skip stvfe_wait for critical object reads (e.g. from LRU)

The exp thread is also critical. It must not starve, otherwise lru
starves also because of hitting OC_F_DYING objects.

src/fellow_storage.c

@@ -414,7 +414,7 @@ stvfe_dskoc_fco(struct worker *wrk,
     const struct stevedore *stv, const struct stvfe *stvfe,
     struct objcore *oc, unsigned crit)
 {
-	struct stvfe_wait_entry *we;
+	struct stvfe_wait_entry *we = NULL;
 	struct fellow_cache_res fcr;
 	struct objcore *refoc;
 	struct fcoc fcoc = {0};
@@ -436,10 +436,12 @@ stvfe_dskoc_fco(struct worker *wrk,
 		return (fcoc);
 	}
 
-	we = stvfe_wait(stvfe->wait, oc->stobj->priv2);
-	if (we == NULL ) {
-		counter = &stvfe->stats->c_dsk_obj_get_coalesce;
-		goto again;
+	if (! crit) {
+		we = stvfe_wait(stvfe->wait, oc->stobj->priv2);
+		if (we == NULL ) {
+			counter = &stvfe->stats->c_dsk_obj_get_coalesce;
+			goto again;
+		}
 	}
 
 	/* we race to get the fco and, if we win the race,
@@ -486,7 +488,8 @@ stvfe_dskoc_fco(struct worker *wrk,
 	stvfe->stats->c_dsk_obj_get++;
 
   out:
-	stvfe_wait_signal(we, oc->stobj->priv2);
+	if (we)
+		stvfe_wait_signal(we, oc->stobj->priv2);
 	return (fcoc);
 }
 
@@ -608,7 +611,7 @@ sfedsk_objfree(struct worker *wrk, struct objcore *dskoc)
 		stvfe->stats->c_dsk_obj_free_thin++;
 	}
 	else if (
-	    (fcoc = stvfe_dskoc_fco(wrk, stv, stvfe, dskoc, 0)).fco != NULL) {
+	    (fcoc = stvfe_dskoc_fco(wrk, stv, stvfe, dskoc, 1)).fco != NULL) {
 		/* attempt to read to free space */
 		fellow_cache_obj_delete(stvfe->fc, fcoc.fco, oh->digest);
 		fcoc_fini(stvfe->fc, &fcoc);