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    SubjectRe: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

    On (01/19/18 13:20), Steven Rostedt wrote:
    [..]
    > I was thinking about this a bit more, and instead of offloading a
    > recursive printk, perhaps its best to simply throttle it. Because the
    > problem may not go away if a printk thread takes over, because the bug
    > is really the printk infrastructure filling the printk buffer keeping
    > printk from ever stopping.

    right. I didn't quite got it how that would help. if we would
    kick_offload every time we add new printks after call_console_drivers(),
    then we can just end up in a kick_offload loop traveling across all CPUs.

    [..]
    > asmlinkage int vprintk_emit(int facility, int level,
    > const char *dict, size_t dictlen,
    > @@ -1849,6 +1918,17 @@ asmlinkage int vprintk_emit(int facility, int level,
    >
    > /* This stops the holder of console_sem just where we want him */
    > logbuf_lock_irqsave(flags);
    > +
    > + if (recursion_check_test()) {
    > + /* A printk happened within a printk at the same context */
    > + if (this_cpu_inc_return(recursion_count) > recursion_max) {
    > + atomic_inc(&recursion_overflow);
    > + logbuf_unlock_irqrestore(flags);
    > + printed_len = 0;
    > + goto out;
    > + }
    > + }

    didn't have time to look at this carefully, but is this possible?

    printks from console_unlock()->call_console_drivers() are redirected
    to printk_safe buffer. we need irq_work on that CPU to flush its
    printk_safe buffer.

    > EXPORT_SYMBOL(vprintk_emit);
    > @@ -2343,9 +2428,14 @@ void console_unlock(void)
    > seen_seq = log_next_seq;
    > }
    >
    > - if (console_seq < log_first_seq) {
    > + if (console_seq < log_first_seq || atomic_read(&recursion_overflow)) {
    > + size_t missed;
    > +
    > + missed = atomic_xchg(&recursion_overflow, 0);
    > + missed += log_first_seq - console_seq;
    > +
    > len = sprintf(text, "** %u printk messages dropped **\n",
    > - (unsigned)(log_first_seq - console_seq));
    > + (unsigned)missed);
    >
    > /* messages are gone, move to first one */
    > console_seq = log_first_seq;

    how are we going to distinguish between lockdep splats, for instance,
    or WARNs from call_console_drivers() -> foo_write(), which are valuable,
    and kmalloc() print outs, which might be less valuable? are we going to
    lose all of them now? then we can do a much simpler thing - steal one
    bit from `printk_context' and use if for a new PRINTK_NOOP_CONTEXT, which
    will be set around call_console_drivers(). vprintk_func() would redirect
    printks to vprintk_noop(fmt, args), which will do nothing.

    -ss

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