Re: Contention on ->i_shared_lock in dup_mmap()

[Alexander Viro <viro@math.psu.edu>]

On Thu, 8 Jun 2000, Manfred Spraul wrote:

> Alexander Viro wrote:
> > 
> > OK, do_syslog() is just plain silly - it's resetting the buffer and code
> > in question looks so:
> >                 spin_lock_irq(&console_lock);
> >                 logged_chars = 0;
> >                 spin_unlock_irq(&console_lock);
> > ... which is for all purposes equivalent to
> >                 if (logged_chars) {
> >                         ...
> >                 }
> > so this one is easy (looks like a klogd silliness).
> > 
> cpu0: do_syslog
> 	logged_chars = 0;
> 
> cpu1: printk()
> 	logged_chars++;
> 
> We really need the spinlock :-(

And? With the current code:

cpu0:					cpu1:
	spin_lock_irq
	logged_chars = 0
	spin_unlock_irq
					spin_lock_irq
					...
					logged_chars++;
	break;

IOW, you have no warranty that upon the exit from do_syslog() you will
have logged_chars == 0 and that's precisely the same as you will get if
you replace the code with
	if (logged_chars) {
		spin_lock_irq(&console_lock);
		logged_chars = 0;
		spin_unlock_irq(&console_lock);
	}
- if logged_chars was non-zero we are getting the same result anyway and
if it was and something was in the middle of a changing it to non-zero -
fine, we just act as if do_syslog() happened before that.

As for the source of contention... beats me. _Probably_ weird mutated
klogd, but I didn't look at the patches RH slapped on it. syslog(5,...) is
damn silly anyway - it's an unavoidable race.

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