Decide a race between obj_get() and lru

we only set stobj->priv after returning from obj_get(), so
assert(oc->stobj->priv == fco) could trigger in the lru thread.

We now set the priv right before inserting into LRU.

src/fellow_cache.c

@@ -4893,6 +4893,10 @@ struct objcore **ocp, uintptr_t priv2, unsigned crit)
 	AN(fcs->fco_infdb);
 	fcs->disk_seg = &fdo->fdo_fds;
 	fellow_cache_seg_transition_locked(lcb, fcs, fcs->state, FCO_INCORE);
+	if (fco->oc) {
+		AZ(fco->oc->stobj->priv);
+		fco->oc->stobj->priv = fco;
+	}
 	fellow_cache_lru_chgbatch_apply(lcb);
 	if (fcs->refcnt > 1)
 		AZ(pthread_cond_broadcast(&fco->cond));
@@ -5494,12 +5498,11 @@ test_fellow_cache_obj_get(struct fellow_cache *fc, uintptr_t priv2,
 	struct objcore *oc1, *toc, ocmem[1];
 	int injcount;
 
-	INIT_OBJ(ocmem, OBJCORE_MAGIC);
-
 	DBG("expect %s", expect == incache ? "incache" : "notincache");
 
 	// baseline, no injection
 	fc_inj_reset();
+	INIT_OBJ(ocmem, OBJCORE_MAGIC);
 	toc = ocmem;
 	fcr = fellow_cache_obj_get(fc, &toc, priv2, 0);
 	if (fcr.status != fcr_ok) {
@@ -5524,6 +5527,7 @@ test_fellow_cache_obj_get(struct fellow_cache *fc, uintptr_t priv2,
 	AN(injcount);
 	while (injcount) {
 		fc_inj_set(injcount);
+		INIT_OBJ(ocmem, OBJCORE_MAGIC);
 		toc = ocmem;
 		fcr = fellow_cache_obj_get(fc, &toc, priv2, 0);
 		AN(toc);
@@ -5535,6 +5539,7 @@ test_fellow_cache_obj_get(struct fellow_cache *fc, uintptr_t priv2,
 	DBG("fco1e %p ref %u", fco, FCO_REFCNT(fco));
 
 	fc_inj_reset();
+	INIT_OBJ(ocmem, OBJCORE_MAGIC);
 	toc = ocmem;
 	fcr = fellow_cache_obj_get(fc, &toc, priv2, 0);
 	DBG("fco2 %p ref %u", fco, FCO_REFCNT(fco));
@@ -5558,8 +5563,6 @@ static void test_fellow_cache_obj_iter_final(
 	struct fcscursor c;
 	int injcount;
 
-	INIT_OBJ(ocmem, OBJCORE_MAGIC);
-
 	AN(fcop);
 	AN(*fcop);
 
@@ -5599,6 +5602,7 @@ static void test_fellow_cache_obj_iter_final(
 		DBG("injcount %d", injcount);
 
 		fc_inj_set(0);
+		INIT_OBJ(ocmem, OBJCORE_MAGIC);
 		toc = ocmem;
 		fcr = fellow_cache_obj_get(fc, &toc, priv2, 0);
 		assert(fcr.status == fcr_ok);
@@ -5621,6 +5625,7 @@ static void test_fellow_cache_obj_iter_final(
 
 	// get back the original number of references
 	for (u = 0; u < refcnt; u++) {
+		INIT_OBJ(ocmem, OBJCORE_MAGIC);
 		toc = ocmem;
 		fcr = fellow_cache_obj_get(fc, &toc, priv2, 0);
 		assert(fcr.status == fcr_ok);

src/fellow_storage.c

@@ -420,7 +420,7 @@ stvfe_dskoc_fco(struct worker *wrk,
 	struct objcore *refoc;
 	struct fcoc fcoc = {0};
 	uint64_t *counter;
-	void *priv, *old;
+	void *priv;
 
 	CHECK_OBJ_NOTNULL(stvfe, STVFE_MAGIC);
 	AN(oc->stobj->priv2);
@@ -477,9 +477,7 @@ stvfe_dskoc_fco(struct worker *wrk,
 		goto out;
 	}
 
-	/* fellow_cache_obj_get() decides the race already */
-	old = __atomic_exchange_n(&oc->stobj->priv, fcoc.fco, __ATOMIC_SEQ_CST);
-	AZ(old);
+	assert(fcoc.fco == oc->stobj->priv);
 	assert(stv == stvfe->dskstv);
 	oc->stobj->stevedore = stvfe->memstv;