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On Tue, May 27, 2008 at 08:03:03AM -0700, Linus Torvalds wrote: > > > 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. I never stop wondering that... > > > 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. SRCU does wait for all CPUs to schedule, and thus already waits for all pre-existing non-preemptable code sequences to finish on all CPUs. > Paul? How hard would it be to make preemptable RCU just honor that classic > RCU behavior? Hmmm... Might not be too hard, I will look into this. Should just be another stage in the rcu_try_flip state machine, along with a few of the changes already in the queue for call_rcu_sched(). But this will only help until preemptible spinlocks arrive, right? Thanx, Paul | ||||||||||||
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