Messages in this thread | | | Date | Wed, 22 May 2013 17:07:07 +0400 | From | Roman Gushchin <> | Subject | Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro |
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On 22.05.2013 16:30, Eric Dumazet wrote: > On Wed, 2013-05-22 at 15:58 +0400, Roman Gushchin wrote: > >> +/* >> + * Same as ACCESS_ONCE(), but used for accessing field of a structure. >> + * The main goal is preventing compiler to store &ptr->field in a register. > > But &ptr->field is a constant during the whole duration of > udp4_lib_lookup2() and could be in a register, in my case field is at > offset 0, and ptr is a parameter (so could be in a 'register') > > The bug you found is that compiler caches the indirection (ptr->field) > into a register, not that compiler stores &ptr->field into a register. > >> + */ >> +#define ACCESS_FIELD_ONCE(PTR, FIELD) (((volatile typeof(*PTR) *)PTR)->FIELD) >> + > > Here we force the compiler to consider ptr as volatile, but semantically > it is not required in rcu_dereference(ptr->field)
Actually, we need to mark an "address of a place" where the field value is located as volatile before dereferencing. I have no idea how to do it in another way, except using multiple casts and offsetof's, but, IMHO, it will be even more complex: ACCESS_ONCE(typeof(&ptr->field)((char*)ptr + offsetof(typeof(*ptr), field)))
> > We want field to be reloaded, not ptr. > > So yes, the patch appears to fix the bug, but it sounds not logical to > me. >
May be we can enhance it by providing better/more detailed comments here? Have you any suggestions?
Thanks, Roman
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