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[94.245.46.49]) by smtp.gmail.com with ESMTPSA id v8sm4248533lfp.85.2020.04.22.03.35.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 22 Apr 2020 03:35:45 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Wed, 22 Apr 2020 12:35:36 +0200 To: "Joel Fernandes (Google)" Cc: linux-kernel@vger.kernel.org, Sebastian Andrzej Siewior , Uladzislau Rezki , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , "Paul E. McKenney" , rcu@vger.kernel.org, Steven Rostedt Subject: Re: [PATCH rcu/dev -fixes 2/4] rcu/tree: Skip entry into the page allocator for PREEMPT_RT Message-ID: <20200422103536.GA16263@pc636> References: <20200420153837.194532-1-joel@joelfernandes.org> <20200420153837.194532-3-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200420153837.194532-3-joel@joelfernandes.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Mon, Apr 20, 2020 at 11:38:35AM -0400, Joel Fernandes (Google) wrote: > To keep kfree_rcu() path working on raw non-preemptible sections, > prevent the optional entry into the allocator as it uses sleeping locks. > In fact, even if the caller of kfree_rcu() is preemptible, this path > still is not, as krcp->lock is a raw spinlock as done in previous > patches. With additional page pre-allocation in the works, hitting this > return is going to be much less likely soon so just prevent it for now > so that PREEMPT_RT does not break. Note that page allocation here is an > optimization and skipping it still makes kfree_rcu() work. > > Cc: Sebastian Andrzej Siewior > Co-developed-by: Uladzislau Rezki > Signed-off-by: Uladzislau Rezki > Signed-off-by: Joel Fernandes (Google) > --- > kernel/rcu/tree.c | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index cf68d3d9f5b81..cd61649e1b001 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3092,6 +3092,18 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, > if (!bnode) { > WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE); > > + /* > + * To keep this path working on raw non-preemptible > + * sections, prevent the optional entry into the > + * allocator as it uses sleeping locks. In fact, even > + * if the caller of kfree_rcu() is preemptible, this > + * path still is not, as krcp->lock is a raw spinlock. > + * With additional page pre-allocation in the works, > + * hitting this return is going to be much less likely. > + */ > + if (IS_ENABLED(CONFIG_PREEMPT_RT)) > + return false; > + > bnode = (struct kfree_rcu_bulk_data *) > __get_free_page(GFP_NOWAIT | __GFP_NOWARN); > } This will not be correctly working by just reverting everything to the "rcu_head path" for CONFIG_PREEMPT_RT kernel. We need to preallocate at least once. I can add caching on top of this series to address it. -- Vlad Rezki