On Thu, Mar 19, 2015 at 01:39:58PM -0400, Steven Rostedt wrote:
> > +void printk_nmi_backtrace_complete(void)
> > +{
> > + struct nmi_seq_buf *s;
> > + int len, cpu, i, last_i;
> > +
> > + /*
> > + * Now that all the NMIs have triggered, we can dump out their
> > + * back traces safely to the console.
> > + */
> > + for_each_possible_cpu(cpu) {
> > + s = &per_cpu(nmi_print_seq, cpu);
> > + last_i = 0;
> > +
> > + len = seq_buf_used(&s->seq);
> > + if (!len)
> > + continue;
> > +
> > + /* Print line by line. */
> > + for (i = 0; i < len; i++) {
> > + if (s->buffer[i] == '\n') {
> > + print_seq_line(s, last_i, i);
> > + last_i = i + 1;
> > + }
> > + }
> > + /* Check if there was a partial line. */
> > + if (last_i < len) {
> > + print_seq_line(s, last_i, len - 1);
> > + pr_cont("\n");
> > + }
> > +
> > + /* Wipe out the buffer ready for the next time around. */
> > + seq_buf_clear(&s->seq);
> > + }
> > +
> > + clear_bit(0, &nmi_print_flag);
> > + smp_mb__after_atomic();
>
> Is this really necessary. What is the mb synchronizing?
>
> [ Added Peter Zijlstra to confirm it's not needed ]
It surely looks suspect; and it lacks a comment, which is a clear sign
its buggy.
Now it if tries to order the accesses to the seqbuf againt the clearing
of the bit one would have expected a _before_ barrier, not an _after_.
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