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Wed, 06 Mar 2019 23:30:33 -0800 (PST) Received: from localhost ([175.223.39.30]) by smtp.gmail.com with ESMTPSA id k9sm7439695pfc.57.2019.03.06.23.30.31 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 06 Mar 2019 23:30:32 -0800 (PST) Date: Thu, 7 Mar 2019 16:30:29 +0900 From: Sergey Senozhatsky To: John Ogness Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Petr Mladek , Sergey Senozhatsky , Steven Rostedt , Daniel Wang , Andrew Morton , Linus Torvalds , Greg Kroah-Hartman , Alan Cox , Jiri Slaby , Peter Feiner , linux-serial@vger.kernel.org, Sergey Senozhatsky Subject: Re: [RFC PATCH v1 19/25] printk: introduce emergency messages Message-ID: <20190307073029.GA489@jagdpanzerIV> References: <20190212143003.48446-1-john.ogness@linutronix.de> <20190212143003.48446-20-john.ogness@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190212143003.48446-20-john.ogness@linutronix.de> User-Agent: Mutt/1.11.3 (2019-02-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On (02/12/19 15:29), John Ogness wrote: [..] > +static bool console_can_emergency(int level) > +{ > + struct console *con; > + > + for_each_console(con) { > + if (!(con->flags & CON_ENABLED)) > + continue; > + if (con->write_atomic && level < emergency_console_loglevel) > + return true; > + if (con->write && (con->flags & CON_BOOT)) > + return true; > + } > + return false; > +} > + > +static void call_emergency_console_drivers(int level, const char *text, > + size_t text_len) > +{ > + struct console *con; > + > + for_each_console(con) { > + if (!(con->flags & CON_ENABLED)) > + continue; > + if (con->write_atomic && level < emergency_console_loglevel) { > + con->write_atomic(con, text, text_len); > + continue; > + } > + if (con->write && (con->flags & CON_BOOT)) { > + con->write(con, text, text_len); > + continue; > + } > + } > +} > + > +static void printk_emergency(char *buffer, int level, u64 ts_nsec, u16 cpu, > + char *text, u16 text_len) > +{ > + struct printk_log msg; > + size_t prefix_len; > + > + if (!console_can_emergency(level)) > + return; > + > + msg.level = level; > + msg.ts_nsec = ts_nsec; > + msg.cpu = cpu; > + msg.facility = 0; > + > + /* "text" must have PREFIX_MAX preceding bytes available */ > + > + prefix_len = print_prefix(&msg, > + console_msg_format & MSG_FORMAT_SYSLOG, > + printk_time, buffer); > + /* move the prefix forward to the beginning of the message text */ > + text -= prefix_len; > + memmove(text, buffer, prefix_len); > + text_len += prefix_len; > + > + text[text_len++] = '\n'; > + > + call_emergency_console_drivers(level, text, text_len); So this iterates the console list and calls consoles' callbacks, but what prevents console driver to be rmmod-ed under us? CPU0 CPU1 printk_emergency() rmmod netcon call_emergency_console_drivers() con_foo->flags & CON_ENABLED == 1 unregister_console(con_foo) con_foo->flags &= ~CON_ENABLED __exit // con_foo gone ? con_foo->write() We use console_lock()/console_trylock() in order to protect the list and console drivers; but this brings scheduler to the picture, with all its locks. Or am I missing something? -ss