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Tue, 24 Mar 2020 19:35:09 -0700 (PDT) Received: from localhost ([2401:fa00:8f:203:5bbb:c872:f2b1:f53b]) by smtp.gmail.com with ESMTPSA id z17sm17198052pff.12.2020.03.24.19.35.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 24 Mar 2020 19:35:08 -0700 (PDT) Date: Wed, 25 Mar 2020 11:35:06 +0900 From: Sergey Senozhatsky To: Zygo Blaxell Cc: Qian Cai , tytso@mit.edu, arnd@arndb.de, gregkh@linuxfoundation.org, sergey.senozhatsky.work@gmail.com, pmladek@suse.com, rostedt@goodmis.org, catalin.marinas@arm.com, will@kernel.org, dan.j.williams@intel.com, peterz@infradead.org, longman@redhat.com, tglx@linutronix.de, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: dmesg -w regression in v5.4.22, bisected, was: Re: [PATCH] char/random: silence a lockdep splat with printk() Message-ID: <20200325023506.GB241329@google.com> References: <1573679785-21068-1-git-send-email-cai@lca.pw> <20200324151359.GF2693@hungrycats.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200324151359.GF2693@hungrycats.org> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000004, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On (20/03/24 11:13), Zygo Blaxell wrote: > On Wed, Nov 13, 2019 at 04:16:25PM -0500, Qian Cai wrote: > > From: Sergey Senozhatsky > > > > Sergey didn't like the locking order, > > > > uart_port->lock -> tty_port->lock > > > > uart_write (uart_port->lock) > > __uart_start > > pl011_start_tx > > pl011_tx_chars > > uart_write_wakeup > > tty_port_tty_wakeup > > tty_port_default > > tty_port_tty_get (tty_port->lock) > > > > but those code is so old, and I have no clue how to de-couple it after > > checking other locks in the splat. There is an onging effort to make all > > printk() as deferred, so until that happens, workaround it for now as a > > short-term fix. > > Starting with v5.4.22 I noticed 'dmesg -w' stopped working on some > machines. dmesg will follow console output for a few seconds, then it > stops. strace indicates dmesg is blocked in read() on the /dev/kmsg fd. > If a new dmesg process starts, it gives messages for a few seconds, > then also stops. rsyslog's kernel logging is similarly affected. > > Bisection points to this patch (now known as > 1b710b1b10eff9d46666064ea25f079f70bc67a8 upstream). I can't reproduce > the problem on a test VM, and some machines are running v5.4.22..v5.4.26 > with no dmesg problems. It seems there is some magic in the startup > sequence of affected machines. This code isn't executed after RNG is > seeded, so it would have to get its bad stuff done before that happens. > > Reverting commit 1b710b1b10eff9d46666064ea25f079f70bc67a8 fixes the > dmesg regression on 5.4.26. It might put the original lockdep bug back, > but on machines running stable kernels, I prefer randomly broken lockdep > over repeatably broken dmesg. This should fix the problem https://lore.kernel.org/lkml/20200303113002.63089-1-sergey.senozhatsky@gmail.com -ss