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Andrew Morton wrote:
>
> George Anzinger wrote:
> >
> > The notion of releasing a spin lock by initializing it seems IMHO, on
> > the face of it, way off. Firstly the protected area is no longer
> > protected which could lead to undefined errors/ crashes and secondly,
> > any future use of spinlocks to control preemption could have a lot of
> > trouble with this, principally because the locker is unknown.
> >
> > In the case at hand, it would seem that an unlocked path to the console
> > is a more correct answer that gives the system a far better chance of
> > actually remaining viable.
> >
>
> Does bust_spinlocks() muck up the preemptive kernel's spinlock
> counting? Would you prefer spin_trylock()/spin_unlock()?
> It doesn't matter - if we call bust_spinlocks() the kernel is
> known to be dead meat and there is a fsck in your near future.

Well, actually this fails just as badly as the locker is not unlocking
and the preemption counts are task local... BUT, see below.
>
> We are still trying to find out why kumon@fujitsu's 8-way is
> crashing on the test10-pre5 sched.c. Looks like it's fixed
> in test11-pre2 but we want to know _why_ it's fixed. And at
> present each time he hits the bug, his printk() deadlocks.
>
> So bust_spinlocks() is a RAS feature :) A very important one -
> it's terrible when your one-in-a-trillion bug happens and there
> are no diagnostics.
>
I agree, this is why, in the preemption patch, we have an "unlocked"
printk. Attached is the relevant portion of the preemption patch for
test9.

I think it still suffers from the console lock, but it is a bit further
down the road.

The patch also illustrates why I am looking for a way to pass var args
to the next function down the line. If I had this the patch would be
WAY simple and would not duplicate the body of printf.

George

> It's a work-in-progress. There are a lot of things which
> can cause printk to deadlock:
>
> - console_lock
> - timerlist_lock
> - global_irq_lock (console code does global_cli)
> - log_wait.lock
> - tasklist_lock (printk does wake_up) (*)
> - runqueue_lock (printk does wake_up)
>
> I'll be proposing a better patch for this in a few days.
>
> (*) Keith: this explains why you can't do a printk() in
> __wake_up_common: printk calls wake_up(). Duh.diff -urP -X patch.exclude linux-2.4.0-test9-kb-rts/kernel/printk.c linux/kernel/printk.c
--- linux-2.4.0-test9-kb-rts/kernel/printk.c Wed Jul 5 11:00:21 2000
+++ linux/kernel/printk.c Thu Nov 2 10:17:20 2000
@@ -312,6 +312,64 @@
return i;
}

+#if defined(CONFIG_KGDB) && defined(CONFIG_PREEMPT)
+asmlinkage int printk_unlocked(const char *fmt, ...)
+{
+ va_list args;
+ int i;
+ char *msg, *p, *buf_end;
+ int line_feed;
+ static signed char msg_level = -1;
+
+ va_start(args, fmt);
+ i = vsprintf(buf + 3, fmt, args); /* hopefully i < sizeof(buf)-4 */
+ buf_end = buf + 3 + i;
+ va_end(args);
+ for (p = buf + 3; p < buf_end; p++) {
+ msg = p;
+ if (msg_level < 0) {
+ if (
+ p[0] != '<' ||
+ p[1] < '0' ||
+ p[1] > '7' ||
+ p[2] != '>'
+ ) {
+ p -= 3;
+ p[0] = '<';
+ p[1] = default_message_loglevel + '0';
+ p[2] = '>';
+ } else
+ msg += 3;
+ msg_level = p[1] - '0';
+ }
+ line_feed = 0;
+ for (; p < buf_end; p++) {
+ log_buf[(log_start+log_size) & LOG_BUF_MASK] = *p;
+ if (log_size < LOG_BUF_LEN)
+ log_size++;
+ else
+ log_start++;
+
+ logged_chars++;
+ if (*p == '\n') {
+ line_feed = 1;
+ break;
+ }
+ }
+ if (msg_level < console_loglevel && console_drivers) {
+ struct console *c = console_drivers;
+ while(c) {
+ if ((c->flags & CON_ENABLED) && c->write)
+ c->write(c, msg, p - msg + line_feed);
+ c = c->next;
+ }
+ }
+ if (line_feed)
+ msg_level = -1;
+ }
+ return i;
+}
+#endif
void console_print(const char *s)
{
struct console *c;
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