Re: 2.6.25-git2: BUG: unable to handle kernel paging request at ffffffffffffffff

[Rafael J. Wysocki]

On Sunday, 20 of April 2008, Linus Torvalds wrote:
> 
> On Sun, 20 Apr 2008, Rafael J. Wysocki wrote:
> > 
> > I've just got the following traces from 2.6.25-git2 on HP nx6325 (64-bit).
> > I think they are related to the hang I described yesterday:
> > 
> > [12844.066757] BUG: unable to handle kernel paging request at ffffffffffffffff
> 
> Something has added a dentry pointer that has the value -1 to the dentry 
> hash list. The access that oopses seems to be the
> 
> 	prefetch(pos->next)
> 
> which is part of hlist_for_each_entry_rcu(), where "pos" is -1.
> 
> I suspect it's an RCU error, ie somebody has released a dentry entry, and 
> free'd it without waiting for the RCU grace period.
> 
> Talking about RCU I also think that whoever did those "rcu_dereference()" 
> macros in <linux/list.h> was insane. It's totally pointless to do 
> "rcu_dereference()" on a local variable. It simply *cannot* make sense. 
> Herbert, Paul, you guys should look at it.
> 
> As far as I can tell, rcu_dereference() should _always_ be done when we 
> access the "next" pointer (except for when prefetching, where we simply 
> don't care).
> 
> Paul? Herbert? Totally untested patch appended.
> 
> NOTE! I do not expect this patch to matter for this oops. There's 
> something else going on there.

Well, it seems that the oops is actually known from -mm:

http://lkml.org/lkml/2008/4/21/55

and something similar was observed with 2.6.25-rc8-mm2.

Thanks,
Rafael
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