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. Perhaps this shows up in proc because the hash chains are short and frequently NULL? Eric _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
