On 03/07/2013 01:05 PM, ebiederm@xxxxxxxxxxxx wrote: > Sasha Levin <sasha.levin@xxxxxxxxxx> writes: > >> On 03/07/2013 12:46 PM, Eric Dumazet wrote: >>> On Thu, 2013-03-07 at 12:36 -0500, Sasha Levin wrote: >>> >>>> Looks like the hlist change is probably the issue, though it specifically >>>> uses: >>>> >>>> #define hlist_entry_safe(ptr, type, member) \ >>>> (ptr) ? hlist_entry(ptr, type, member) : NULL >>>> >>>> I'm still looking at the code in question and it's assembly, but I can't >>>> figure out what's going wrong. I was also trying to see what's so special >>>> about this loop in find_pid_ns as opposed to the rest of the kernel code >>>> that uses hlist_for_each_entry_rcu() but couldn't find out why. >>>> >>>> Is it somehow possible that if we rcu_dereference_raw() the same thing twice >>>> inside the same rcu_read_lock() section we'll get different results? That's >>>> really the only reason for this crash that comes to mind at the moment, very >>>> unlikely - but that's all I have right now. >>>> >>> >>> Yep >>> >>> #define hlist_entry_safe(ptr, type, member) \ >>> (ptr) ? hlist_entry(ptr, type, member) : NULL >>> >>> Is not safe, as ptr can be evaluated twice, and thats not good at all... >> >> ptr is being evaluated twice, but in this case this is an >> rcu_dereference_raw() value within the same rcu_read_lock() section. >> >> Is it still problematic? > > Definitely. > > Head in this instance the expression: &pid_hash[pid_hashfn(nr, ns)] > > And the crash clearly shows that when hilst_entry is being evaluated the > HEAD is NULL. Okay, I'm even more confused now. The expression in question is: hlist_entry_safe(rcu_dereference_bh(hlist_first_rcu(head))) You're saying that "rcu_dereference_bh(hlist_first_rcu(head))" can change between the two evaluations we do. That would mean that 'head' has changed in between, right? In that case, the list itself has changed - which means that RCU has changed the list underneath us. hlist_first_rcu() doesn't have any side-effects, it doesn't modify the list whatsoever, so the only thing that can change is 'head'. Why is it allowed to change if the list is protected by RCU? Thanks, Sasha _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
