Messages in this thread | | | Date | Sat, 25 May 2013 04:37:15 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro |
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On Wed, May 22, 2013 at 11:17:46PM +0400, Roman Gushchin wrote: > On 22.05.2013 21:45, Paul E. McKenney wrote: > >On Wed, May 22, 2013 at 05:07:07PM +0400, Roman Gushchin wrote: > >>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))) > > Probably I miss one more ACCESS_ONCE() in this expression. Should be something like > ACCESS_ONCE(typeof(&ptr->field)((char*)ACCESS_ONCE(ptr) + offsetof(typeof(*ptr), field))) . > But this is not a working example, just an illustration against using ACCESS_ONCE() here. > > >Why not just ACCESS_ONCE(ptr->field)? Or if it is the thing that > >ptr->field points to that is subject to change, ACCESS_ONCE(*ptr->field)? > > > >Or rcu_dereference(ptr->field), as appropriate? > > It's not enough. So is the code now, and it doesn't work as expected. > You can't write (ptr->field) without ptr being marked as a volatile pointer. > > I try to explain the problem once more from scratch: > > 1) We have the following pseudo-code (based on udp4_lib_lookup2()): > > static void some_func(struct hlist_nulls_head *head) { > struct hlist_nulls_node *node; > > begin: > for (node = rcu_dereference(head->first); > !is_nulls(node) & ...; > node = rcu_dereference(node->next)) { > <...> > } > > if (restart_condition) > goto begin; > > 2) A problem occurs when restart_condition is true and we jump to the begin label. > We do not recalculate (head + offsetof(head, first)) address, we just dereference > again the OLD (head->first) pointer. So, we get a node, that WAS the first node in a > previous time instead of current first node. That node can be dead, or, for instance, > can be a head of another chain.
OK, this is what I was referring to when I said that the RCU list macros assume that the list header is static (or equivalently, appropriately protected).
With some_func() as written above, you would need to return some sort of failure indication from some_func(), and have the caller refetch head. Otherwise, as far as gcc is concerned, the value of the parameter head is constant throughout some_func().
> It is correct from gcc's point of view, since it doesn't expect the head pointer > to change, and this address is just (head + constant offset).
Agreed.
How does the caller calculate the value to pass in through the argument "head"?
> 3) If we start with a wrong first element, restart_condition can be always true, so, > we get an infinite loop. In any case, we do not scan the whole (socket) chain, > as expected.
Agreed.
> This scenario is absolutely real and causes our DNS servers to hang > sometimes under high load.
I completely believe that such a hang could happen.
> Note, that there are no problems if we don't restart a loop. Also, it is highly > dependent on gcc options, and the code in the body of the loop. Even small changes > in the code (like adding debugging print) preventing reproducing of the issue.
Again, I believe that your retry logic needs to extend back into the calling function for your some_func() example above.
Thanx, Paul
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