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[209.85.220.41]) by mx.google.com with SMTPS id x24sor21004247pgk.36.2019.07.22.08.14.42 for (Google Transport Security); Mon, 22 Jul 2019 08:14:42 -0700 (PDT) Received-SPF: pass (google.com: domain of joel@joelfernandes.org designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; Authentication-Results: mx.google.com; dkim=pass header.i=@joelfernandes.org header.s=google header.b=XQ0f5nx8; spf=pass (google.com: domain of joel@joelfernandes.org designates 209.85.220.41 as permitted sender) smtp.mailfrom=joel@joelfernandes.org DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=i8M6AK4de9Ik8pIGK7ZSAo/fkLcx89Rz2FU4ueU7AtI=; b=XQ0f5nx8fa3pklvdGG2u9IOZZJTUyDUTYVVk+sNJ3SCJPR77BhV8NbHcipLbVy1xvT Slg0Se3QHTC0ZyO53T8oEMnYmpMj/IpwJ3J26/3ycCvlPIjGxwvchFbXcMJqk0o/mpqq 1Yt0CuEo1ILTMuVgHTe5nTzixjn08mjGXwYpQ= X-Google-Smtp-Source: APXvYqydSiyMRzj6QXFVzUWNzUL0LUUvnGotJgLD9n9VUjcgfggnl8W+/75TLlsXhUbwOLV6q6l9PQ== X-Received: by 2002:a63:1310:: with SMTP id i16mr71092692pgl.187.1563808481910; Mon, 22 Jul 2019 08:14:41 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id h1sm51944925pfg.55.2019.07.22.08.14.40 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Mon, 22 Jul 2019 08:14:40 -0700 (PDT) Date: Mon, 22 Jul 2019 11:14:39 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: Matthew Wilcox , "Michael S. Tsirkin" , 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: <20190722151439.GA247639@google.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190721233113.GV14271@linux.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: [snip] > > 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. Other than what Paul already mentioned about deadlocks, I am not sure if this would even work for all cases since call_rcu() has to wait for a grace period. So, if the number of outstanding requests are higher than a certain amount, then you *still* have to wait for some RCU configurations for the grace period duration and cannot just execute the callback in-line. Did I miss something? Can waiting in-line for a grace period duration be tolerated in the vhost case? thanks, - Joel