Re: 2.6.26-rc4: RIP find_pid_ns+0x6b/0xa0

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, 27 May 2008, Oleg Nesterov wrote:

> On 05/27, Alexey Dobriyan wrote:
> >
> > PREEMPT_RCU is in use, again.

I do wonder if PREEMPT_RCU is broken.

> > 0xffffffff802447cb is in find_pid_ns (kernel/pid.c:297).
> > 292             struct hlist_node *elem;
> > 293             struct upid *pnr;
> > 294
> > 295             hlist_for_each_entry_rcu(pnr, elem,
> > 296                             &pid_hash[pid_hashfn(nr, ns)], pid_chain)
> > 297                     if (pnr->nr == nr && pnr->ns == ns)

> > general protection fault: 0000 [2] PREEMPT SMP DEBUG_PAGEALLOC
> > RDX: 6b6b6b6b6b6b6b6b RSI: ffffffff80566760 RDI: 0000000000003cef

That repeated 0x6b is POISON_FREE, and the code is

	cmp    -0x10(%rdx),%edi

which is the load of "pnr->nr". So 'pnr' has been free'd.

On Tue, 27 May 2008, Oleg Nesterov wrote:
> 
> Is this reproducible?
> 
> In theory find_pid() is not safe without rcu_read_lock() if CONFIG_PREEMPT_RCU.
> But we have a lot of "read_lock(tasklist_lock) + find_pid()", this was legal
> and documented. It was actually broken, but happened to work because read_lock()
> implied rcu_read_lock().
> 
> Could you look at
> 
> 	[PATCH] fix tasklist + find_pid() with CONFIG_PREEMPT_RCU
> 	http://marc.info/?t=120162615300012
> 
> ?
> 
> I am not sure this is the actual reason though, the race is very unlikely.

That is a *very* unlikely race, especially as that bad_fork_free_pid case 
would only happen if pid_ns_prepare_proc() fails. And if it fails, it's 
still very unlikely to hit, I think.

That said, it does smell like a bug. But I *really* would be much much 
happier if even SRCU at least waited for a grace period, so that it would 
always be safe to just disable preemption for a "rcu_read_lock()". That 
way, things that take spinlocks are safe even with SRCU.

Paul? How hard would it be to make preemptable RCU just honor that classic 
RCU behavior?

			Linus