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[90.233.218.141]) by smtp.gmail.com with ESMTPSA id 38308e7fff4ca-2ec05c05f56sm2099851fa.42.2024.06.13.06.06.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 13 Jun 2024 06:06:57 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Thu, 13 Jun 2024 15:06:54 +0200 To: "Paul E. McKenney" , Vlastimil Babka Subject: Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache_free callback Message-ID: References: <20240609082726.32742-1-Julia.Lawall@inria.fr> <20240612143305.451abf58@kernel.org> <80e03b02-7e24-4342-af0b-ba5117b19828@paulmck-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <80e03b02-7e24-4342-af0b-ba5117b19828@paulmck-laptop> X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Jason A. Donenfeld" , kvm@vger.kernel.org, Neil Brown , kernel-janitors@vger.kernel.org, Olga Kornievskaia , Dai Ngo , Christophe Leroy , coreteam@netfilter.org, "Naveen N. Rao" , Jakub Kicinski , linux-trace-kernel@vger.kernel.org, bridge@lists.linux.dev, ecryptfs@vger.kernel.org, Nicholas Piggin , linux-can@vger.kernel.org, linux-block@vger.kernel.org, Mathieu Desnoyers , Vlastimil Babka , Tom Talpey , linux-nfs@vger.kernel.org, netdev@vger.kernel.org, Lai Jiangshan , linux-kernel@vger.kernel.org, Julia Lawall , netfilter-devel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, wireguard@lists.zx2c4.com Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Thu, Jun 13, 2024 at 05:47:08AM -0700, Paul E. McKenney wrote: > On Thu, Jun 13, 2024 at 01:58:59PM +0200, Jason A. Donenfeld wrote: > > On Wed, Jun 12, 2024 at 03:37:55PM -0700, Paul E. McKenney wrote: > > > On Wed, Jun 12, 2024 at 02:33:05PM -0700, Jakub Kicinski wrote: > > > > On Sun, 9 Jun 2024 10:27:12 +0200 Julia Lawall wrote: > > > > > Since SLOB was removed, it is not necessary to use call_rcu > > > > > when the callback only performs kmem_cache_free. Use > > > > > kfree_rcu() directly. > > > > > > > > > > The changes were done using the following Coccinelle semantic patch. > > > > > This semantic patch is designed to ignore cases where the callback > > > > > function is used in another way. > > > > > > > > How does the discussion on: > > > > [PATCH] Revert "batman-adv: prefer kfree_rcu() over call_rcu() with free-only callbacks" > > > > https://lore.kernel.org/all/20240612133357.2596-1-linus.luessing@c0d3.blue/ > > > > reflect on this series? IIUC we should hold off.. > > > > > > We do need to hold off for the ones in kernel modules (such as 07/14) > > > where the kmem_cache is destroyed during module unload. > > > > > > OK, I might as well go through them... > > > > > > [PATCH 01/14] wireguard: allowedips: replace call_rcu by kfree_rcu for simple kmem_cache_free callback > > > Needs to wait, see wg_allowedips_slab_uninit(). > > > > Also, notably, this patch needs additionally: > > > > diff --git a/drivers/net/wireguard/allowedips.c b/drivers/net/wireguard/allowedips.c > > index e4e1638fce1b..c95f6937c3f1 100644 > > --- a/drivers/net/wireguard/allowedips.c > > +++ b/drivers/net/wireguard/allowedips.c > > @@ -377,7 +377,6 @@ int __init wg_allowedips_slab_init(void) > > > > void wg_allowedips_slab_uninit(void) > > { > > - rcu_barrier(); > > kmem_cache_destroy(node_cache); > > } > > > > Once kmem_cache_destroy has been fixed to be deferrable. > > > > I assume the other patches are similar -- an rcu_barrier() can be > > removed. So some manual meddling of these might be in order. > > Assuming that the deferrable kmem_cache_destroy() is the option chosen, > agreed. > void kmem_cache_destroy(struct kmem_cache *s) { int err = -EBUSY; bool rcu_set; if (unlikely(!s) || !kasan_check_byte(s)) return; cpus_read_lock(); mutex_lock(&slab_mutex); rcu_set = s->flags & SLAB_TYPESAFE_BY_RCU; s->refcount--; if (s->refcount) goto out_unlock; err = shutdown_cache(s); WARN(err, "%s %s: Slab cache still has objects when called from %pS", __func__, s->name, (void *)_RET_IP_); ... cpus_read_unlock(); if (!err && !rcu_set) kmem_cache_release(s); } so we have SLAB_TYPESAFE_BY_RCU flag that defers freeing slab-pages and a cache by a grace period. Similar flag can be added, like SLAB_DESTROY_ONCE_FULLY_FREED, in this case a worker rearm itself if there are still objects which should be freed. Any thoughts here? -- Uladzislau Rezki