Re: [PATCH/RFC v5 4/5]: core: Add dump device to call on oopses and panics

David Woodhouse
2009-11-30T10:23:31Z

On Mon, 2009-11-30 at 10:51 +0200, Artem Bityutskiy wrote:
> On Mon, 2009-11-30 at 08:46 +0100, Jörn Engel wrote:
> > On Mon, 30 November 2009 09:27:51 +0200, Artem Bityutskiy wrote:
> > >
> > > To me it looks like 'log_end' is not supposed to wrap. What makes you
> > > think it can? In which cases it can?
> >
> > It is a 32bit variable. Would do you expect happens once you reach
> > 0xffffffff and add 1?
>
> Yes, now I see log_end is an ever increasing variable.
>
> How about this patch on top of the existing one (untested):

I suspect you should be modelling this more closely on the code in
do_syslog() for case 3. The end is at (log_end & LOG_BUF_MASK), and you
have (logged_chars) to write.

This mean that you won't write messages to the log which were 'cleared'
by 'dmesg -c', but that's acceptable, I think.

Why are you setting s1 to "" in the case where l1 is zero, btw? Can't it
be NULL?

diff --git a/kernel/printk.c b/kernel/printk.c
index f711b99..d150c57 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -1486,26 +1486,33 @@ static const char *kmsg_to_str(enum kmsg_dump_reason reason)
*/
void kmsg_dump(enum kmsg_dump_reason reason)
{
- unsigned long len = ACCESS_ONCE(log_end);
+ unsigned long end;
+ unsigned chars;
struct kmsg_dumper *dumper;
const char *s1, *s2;
unsigned long l1, l2;
unsigned long flags;

- s1 = "";
- l1 = 0;
- s2 = log_buf;
- l2 = len;
-
- /* Have we rotated around the circular buffer? */
- if (len > log_buf_len) {
- unsigned long pos = len & LOG_BUF_MASK;
+ /* Theoretically, the log could move on after we do this, but
+ there's not a log we can do about that. The new messages
+ will overwrite the start of what we dump. */
+ spin_lock_irqsave(&logbuf_lock, flags);
+ end = log_end & LOG_BUF_MASK;
+ chars = logged_chars;
+ spin_unlock_irqrestore(&logbuf_lock, flags);

- s1 = log_buf + pos;
- l1 = log_buf_len - pos;
+ if (logged_chars > end) {
+ s1 = log_buf + log_buf_len - logged_chars + end;
+ l1 = logged_chars - end;

s2 = log_buf;
- l2 = pos;
+ l2 = end;
+ } else {
+ s1 = "";
+ l1 = 0;
+
+ s2 = log_buf + end - logged_chars;
+ l2 = logged_chars;
}

if (!spin_trylock_irqsave(&dump_list_lock, flags)) {

--
David Woodhouse Open Source Technology Centre
David.Woodhouse@intel.com Intel Corporation