On Thu, Mar 07, 2013 at 12:50:47PM -0500, Sasha Levin wrote: > 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? Sigh! Eric is right -- if "ptr" involves side effects, evaluating it twice is completely broken. Thanx, Paul _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
