Re: [PATCH] race condition in procfs
From: Grzegorz Nosek (grzegorz.nosek_at_gmail.com)
Date: 11/30/05
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Date: Wed, 30 Nov 2005 15:41:40 +0100 To: Steven Rostedt <rostedt@goodmis.org>
2005/11/29, Steven Rostedt <rostedt@goodmis.org>:
>
> What you are showing, would probably show up by others if this were a
> vanilla kernel issue. I don't have an 8 way machine, just 2 way, but the
> vanilla kernel is being used on many 8 ways out there, so I think you are
> right that this _is_ a vserver issue.
Yeah, I guess so. I also noticed that running an older build (w/o ACPI
so it sees only 2 CPUs due to lack of HT - it's a dual Xeon HT machine
so there are 4 logical CPUs) seems a bit more stable, but it happens
there too.
>
> Unless, of course, that the vserver is producing an obscure race in the
> vanilla kernel that normal operations would seldom have. Just like the
> PREEMPT_RT patch has discovered many race conditions that were in the
> vanilla kernel but were not often a problem.
>
I'm not using preemption. What made me just stare in wonder was when I
added a check in do_task_stat at the very beginning to the effect of:
if (!task) {
printk(...);
return -ENOENT;
}
/* dereference task as usual */
I *still* got the oops (and no message got logged). So either it is
used before the entry point (there is an occurrence of
sizeof(task->comm) but that should be statically determined by the
compiler, right?) or it is set to NULL in some magical way between the
check and usage (yep, it's still a race but the window should be
smaller I think).
The only place I can find a proc_inode.task field set to NULL is in
proc_pid_make_inode(). However, it is set to the value of task
parameter just a few instructions later. Am I right? Or can
proc_pid_make_inode get passed a NULL pointer?
I'm lost. Any assistance will be invaluable.
Best regards,
Grzegorz Nosek
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