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[66.187.233.206]) by smtp.gmail.com with ESMTPSA id 207sm8926467qkh.33.2019.10.07.08.33.27 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 07 Oct 2019 08:33:28 -0700 (PDT) Message-ID: <1570462407.5576.292.camel@lca.pw> Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk() From: Qian Cai To: Michal Hocko Cc: Petr Mladek , akpm@linux-foundation.org, sergey.senozhatsky.work@gmail.com, rostedt@goodmis.org, peterz@infradead.org, linux-mm@kvack.org, john.ogness@linutronix.de, david@redhat.com, linux-kernel@vger.kernel.org Date: Mon, 07 Oct 2019 11:33:27 -0400 In-Reply-To: <20191007151237.GP2381@dhcp22.suse.cz> 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> 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: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2019-10-07 at 17:12 +0200, Michal Hocko wrote: > On Mon 07-10-19 10:59:10, Qian Cai wrote: > [...] > > It is almost impossible to eliminate all the indirect call chains from > > console_sem/console_owner_lock to zone->lock because it is too normal that > > something later needs to allocate some memory dynamically, so as long as it > > directly call printk() with zone->lock held, it will be in trouble. > > Do you have any example where the console driver really _has_ to > allocate. Because I have hard time to believe this is going to work at > all as the atomic context doesn't allow to do any memory reclaim and > such an allocation would be too easy to fail so the allocation cannot > really rely on it. I don't know how to explain to you clearly, but let me repeat again one last time. There is no necessary for console driver directly to allocate considering this example, CPU0: CPU1: CPU2: CPU3: console_sem->lock zone->lock pi->lock pi->lock rq_lock rq->lock zone->lock console_sem->lock Here it only need someone held the rq_lock and allocate some memory. There is also true for port_lock. Since the deadlock could involve a lot of CPUs and a longer lock chain, it is impossible to predict which one to allocate some memory while held a lock could end up with the same problematic lock chain. > > So again, crippling the MM code just because of lockdep false possitives > or a broken console driver sounds like a wrong way to approach the > problem. > > > [  297.425964] -> #1 (&port_lock_key){-.-.}: > > [  297.425967]        __lock_acquire+0x5b3/0xb40 > > [  297.425967]        lock_acquire+0x126/0x280 > > [  297.425968]        _raw_spin_lock_irqsave+0x3a/0x50 > > [  297.425969]        serial8250_console_write+0x3e4/0x450 > > [  297.425970]        univ8250_console_write+0x4b/0x60 > > [  297.425970]        console_unlock+0x501/0x750 > > [  297.425971]        vprintk_emit+0x10d/0x340 > > [  297.425972]        vprintk_default+0x1f/0x30 > > [  297.425972]        vprintk_func+0x44/0xd4 > > [  297.425973]        printk+0x9f/0xc5 > > [  297.425974]        register_console+0x39c/0x520 > > [  297.425975]        univ8250_console_init+0x23/0x2d > > [  297.425975]        console_init+0x338/0x4cd > > [  297.425976]        start_kernel+0x534/0x724 > > [  297.425977]        x86_64_start_reservations+0x24/0x26 > > [  297.425977]        x86_64_start_kernel+0xf4/0xfb > > [  297.425978]        secondary_startup_64+0xb6/0xc0 > > This is an early init code again so the lockdep sounds like a false > possitive to me. This is just a tip of iceberg to show the lock dependency, console_owner --> port_lock_key which could easily happen everywhere with a simple printk().