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[156.34.55.100]) by smtp.gmail.com with ESMTPSA id x8sm17451291qkl.27.2019.07.22.06.41.52 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 22 Jul 2019 06:41:52 -0700 (PDT) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1hpYZc-0003Vr-7E; Mon, 22 Jul 2019 10:41:52 -0300 Date: Mon, 22 Jul 2019 10:41:52 -0300 From: Jason Gunthorpe To: "Paul E. McKenney" Cc: "Michael S. Tsirkin" , Matthew Wilcox , aarcange@redhat.com, akpm@linux-foundation.org, christian@brauner.io, davem@davemloft.net, ebiederm@xmission.com, elena.reshetova@intel.com, guro@fb.com, hch@infradead.org, james.bottomley@hansenpartnership.com, jasowang@redhat.com, jglisse@redhat.com, keescook@chromium.org, ldv@altlinux.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-parisc@vger.kernel.org, luto@amacapital.net, mhocko@suse.com, mingo@kernel.org, namit@vmware.com, peterz@infradead.org, syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk, wad@chromium.org Subject: Re: RFC: call_rcu_outstanding (was Re: WARNING in __mmdrop) Message-ID: <20190722134152.GA13013@ziepe.ca> References: <0000000000008dd6bb058e006938@google.com> <000000000000964b0d058e1a0483@google.com> <20190721044615-mutt-send-email-mst@kernel.org> <20190721081933-mutt-send-email-mst@kernel.org> <20190721131725.GR14271@linux.ibm.com> <20190721210837.GC363@bombadil.infradead.org> <20190721233113.GV14271@linux.ibm.com> <20190722035042-mutt-send-email-mst@kernel.org> <20190722115149.GY14271@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190722115149.GY14271@linux.ibm.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 22, 2019 at 04:51:49AM -0700, Paul E. McKenney wrote: > > > > Would it make sense to have call_rcu() check to see if there are many > > > > outstanding requests on this CPU and if so process them before returning? > > > > That would ensure that frequent callers usually ended up doing their > > > > own processing. > > > > > > Unfortunately, no. Here is a code fragment illustrating why: That is only true in the general case though, kfree_rcu() doesn't have this problem since we know what the callback is doing. In general a caller of kfree_rcu() should not need to hold any locks while calling it. We could apply the same idea more generally and have some 'call_immediate_or_rcu()' which has restrictions on the caller's context. I think if we have some kind of problem here it would be better to handle it inside the core code and only require that callers use the correct RCU API. I can think of many places where kfree_rcu() is being used under user control.. Jason