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SubjectRe: [RFC PATCH v1 19/25] printk: introduce emergency messages
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

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