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[66.187.233.206]) by smtp.gmail.com with ESMTPSA id e7sm9048646qtb.94.2019.10.08.06.23.53 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 08 Oct 2019 06:23:53 -0700 (PDT) Message-ID: <1570541032.5576.297.camel@lca.pw> Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk() From: Qian Cai To: Steven Rostedt , Petr Mladek Cc: Michal Hocko , sergey.senozhatsky.work@gmail.com, peterz@infradead.org, linux-mm@kvack.org, john.ogness@linutronix.de, akpm@linux-foundation.org, david@redhat.com, linux-kernel@vger.kernel.org Date: Tue, 08 Oct 2019 09:23:52 -0400 In-Reply-To: <20191008091349.6195830d@gandalf.local.home> References: <1570228005-24979-1-git-send-email-cai@lca.pw> <20191007143002.l37bt2lzqtnqjqxu@pathway.suse.cz> <1570460350.5576.290.camel@lca.pw> <20191007151237.GP2381@dhcp22.suse.cz> <1570462407.5576.292.camel@lca.pw> <20191008081510.ptwmb7zflqiup5py@pathway.suse.cz> <20191008091349.6195830d@gandalf.local.home> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.22.6 (3.22.6-10.el7) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2019-10-08 at 09:13 -0400, Steven Rostedt wrote: > On Tue, 8 Oct 2019 10:15:10 +0200 > Petr Mladek wrote: > > > There are basically three possibilities: > > > > 1. Do crazy exercises with locks all around the kernel to > > avoid the deadlocks. It is usually not worth it. And > > it is a "whack a mole" approach. > > > > 2. Use printk_deferred() in problematic code paths. It is > > a "whack a mole" approach as well. And we would end up > > with printk_deferred() used almost everywhere. > > > > 3. Always deffer the console handling in printk(). This would > > help also to avoid soft lockups. Several people pushed > > against this last few years because it might reduce > > the chance to see the message in case of system crash. > > > > As I said, there has finally been agreement to always do > > the offload few weeks ago. John Ogness is working on it. > > So we might have the systematic solution for these deadlocks > > rather sooner than later. > > Another solution is to add the printk_deferred() in these places that > cause lockdep splats, and when John's work is done, it would be easy to > grep for them and remove them as they would no longer be needed. > > This way we don't play whack-a-mole forever (only until we have a > proper solution) and everyone is happy that we no longer have these > false positive or I-don't-care lockdep splats which hide real lockdep > splats because lockdep shuts off as soon as it discovers its first > splat. I feel like that is what I trying to do, but there seems a lot of resistances with that approach where pragmatism met with perfectionism.