On 2019-03-08, Petr Mladek <[email protected]> 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. > > Great catch!
Yes, thanks! > I think that it is doable to guard the list using RCU. I think it would be enough to take the prb_cpulock when modifying the console linked list. That will keep printk_emergency() out until the list has been updated. (registering/unregistering consoles is not something that happens often.) John Ogness
