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SubjectRe: [RFC PATCH v1 08/25] printk: add ring buffer and kthread
On (02/12/19 15:29), John Ogness wrote:
[..]
> + /* the printk kthread never exits */
> + for (;;) {
> + ret = prb_iter_wait_next(&iter, buf,
> + PRINTK_RECORD_MAX, &master_seq);
> + if (ret == -ERESTARTSYS) {
> + continue;
> + } else if (ret < 0) {
> + /* iterator invalid, start over */
> + prb_iter_init(&iter, &printk_rb, NULL);
> + continue;
> + }
> +
> + msg = (struct printk_log *)buf;
> + format_text(msg, master_seq, ext_text, &ext_len, text,
> + &len, printk_time);
> +
> + console_lock();
> + if (len > 0 || ext_len > 0) {
> + call_console_drivers(ext_text, ext_len, text, len);
> + boot_delay_msec(msg->level);
> + printk_delay();
> + }
> + console_unlock();
> + }

This, theoretically, creates a whole new world of possibilities for
console drivers. Now they can do GFP_KERNEL allocations and stall
printk_kthread during OOM; or they can explicitly reschedule from
->write() callback (via console_conditional_schedule()) because
console_lock() sets console_may_schedule.

It's one thing to do cond_resched() (or to let preemption to take over)
after call_console_drivers() (when we are done printing a message to all
console drivers) and another thing to let preemption to take over while
we are printing a messages to the consoles. It probably would make sense
to disable preemption around call_console_drivers().

-ss

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