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SubjectRe: [RFC PATCH v1 19/25] printk: introduce emergency messages
On Thu 2019-03-07 16:30:29, Sergey Senozhatsky wrote:
> 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.

Great catch!

I think that it is doable to guard the list using RCU.

Best Regards,
Petr

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