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    SubjectRe: [RFC PATCH v1 08/25] printk: add ring buffer and kthread
    On (03/07/19 13:06), John Ogness wrote:
    > On 2019-03-04, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> 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.
    >
    > This was the intention.

    This can stall the entire printing pipeline

    OOM -> printk_kthread() -> console_lock() -> con_foo() -> kmalloc(GFP_KERNEL) -> OOM

    > Although, as I mentioned in a previous response[0], perhaps we should
    > not loosen the requirements on write().

    Right. Console drivers better stay restricted; very restricted.

    > It is exactly that disable_preempt() that is so harmful for realtime tasks.

    I'll reply in another email (later today, or tomorrow).

    -ss

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