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SubjectRe: [RFC PATCH v1 19/25] printk: introduce emergency messages
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On 2019-03-08, Petr Mladek <pmladek@suse.com> 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

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