Messages in this thread | | | Date | Tue, 15 Jul 2008 12:51:23 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [patch 01/15] Kernel Tracepoints |
| |
* Peter Zijlstra (peterz@infradead.org) wrote: > On Tue, 2008-07-15 at 12:08 -0400, Mathieu Desnoyers wrote: > > * Peter Zijlstra (peterz@infradead.org) wrote: > > > On Tue, 2008-07-15 at 11:22 -0400, Mathieu Desnoyers wrote: > > > > * Peter Zijlstra (peterz@infradead.org) wrote: > > > > > > > > > > I'm confused by the barrier games here. > > > > > > > > > > Why not: > > > > > > > > > > void **it_func; > > > > > > > > > > preempt_disable(); > > > > > it_func = rcu_dereference((tp)->funcs); > > > > > if (it_func) { > > > > > for (; *it_func; it_func++) > > > > > ((void(*)(proto))(*it_func))(args); > > > > > } > > > > > preempt_enable(); > > > > > > > > > > That is, why can we skip the barrier when !it_func? is that because at > > > > > that time we don't actually dereference it_func and therefore cannot > > > > > observe stale data? > > > > > > > > > > > > > Exactly. I used the implementation of rcu_assign_pointer as a hint that > > > > we did not need barriers when setting the pointer to NULL, and thus we > > > > should not need the read barrier when reading the NULL pointer, because > > > > it references no data. > > > > > > > > #define rcu_assign_pointer(p, v) \ > > > > ({ \ > > > > if (!__builtin_constant_p(v) || \ > > > > ((v) != NULL)) \ > > > > smp_wmb(); \ > > > > (p) = (v); \ > > > > }) > > > > > > Yeah, I saw that,.. made me wonder. It basically assumes that when we > > > write: > > > > > > rcu_assign_pointer(foo, NULL); > > > > > > foo will not be used as an index or offset. > > > > > > I guess Paul has thought it through and verified all in-kernel use > > > cases, but it still makes me feel unconfortable. > > > > > > > #define rcu_dereference(p) ({ \ > > > > typeof(p) _________p1 = ACCESS_ONCE(p); \ > > > > smp_read_barrier_depends(); \ > > > > (_________p1); \ > > > > }) > > > > > > > > But I think you are right, since we are already in unlikely code, using > > > > rcu_dereference as you do is better than my use of read barrier depends. > > > > It should not change anything in the assembly result except on alpha, > > > > where the read_barrier_depends() is not a nop. > > > > > > > > I wonder if there would be a way to add this kind of NULL pointer case > > > > check without overhead in rcu_dereference() on alpha. I guess not, since > > > > the pointer is almost never known at compile-time. And I guess Paul must > > > > already have thought about it. The only case where we could add this > > > > test is when we know that we have a if (ptr != NULL) test following the > > > > rcu_dereference(); we could then assume the compiler will merge the two > > > > branches since they depend on the same condition. > > > > > > I remember seeing a thread about all this special casing NULL, but have > > > never been able to find it again - my google skillz always fail me. > > > > > > Basically it doesn't work if you use the variable as an index/offset, > > > because in that case 0 is a valid offset and you still generate a data > > > dependency. > > > > > > IIRC the conclusion was that the gains were too small to spend more time > > > on it, although I would like to hear about the special case in > > > rcu_assign_pointer. > > > > > > /me goes use git blame.... > > > > > > > Actually, we could probably do the following, which also adds an extra > > coherency check about non-NULL pointer assumptions : > > > > #ifdef CONFIG_RCU_DEBUG /* this would be new */ > > #define DEBUG_RCU_BUG_ON(x) BUG_ON(x) > > #else > > #define DEBUG_RCU_BUG_ON(x) > > #endif > > > > #define rcu_dereference(p) ({ \ > > typeof(p) _________p1 = ACCESS_ONCE(p); \ > > if (p != NULL) \ > > smp_read_barrier_depends(); \ > > (_________p1); \ > > }) > > > > #define rcu_dereference_non_null(p) ({ \ > > typeof(p) _________p1 = ACCESS_ONCE(p); \ > > DEBUG_RCU_BUG_ON(p == NULL); \ > > smp_read_barrier_depends(); \ > > (_________p1); \ > > }) > > > > The use-case where rcu_dereference() would be used is when it is > > followed by a null pointer check (grepping through the sources shows me > > this is a very very common case). In rare cases, it is assumed that the > > pointer is never NULL and it is used just after the rcu_dereference. It > > those cases, the extra test could be saved on alpha by using > > rcu_dereference_non_null(p), which would check the the pointer is indeed > > never NULL under some debug kernel configuration. > > > > Does it make sense ? > > This would break the case where the dereferenced variable is used as an > index/offset where 0 is a valid value and still generates data > dependencies. > > So if with your new version we do: > > i = rcu_dereference(foo); > j = table[i]; > > which translates into: > > i = ACCESS_ONCE(foo); > if (i) > smp_read_barrier_depends(); > j = table[i]; > > which when i == 0, would fail to do the barrier and can thus cause j to > be a wrong value. > > Sadly I'll have to defer to Paul to explain exactly how that can happen > - I always get my head in a horrible twist with this case. >
I completely agree with you. However, given the current rcu_assign_pointer() implementation, we already have this problem. My proposal assumes the current rcu_assign_pointer() behavior is correct and that those are never ever used for index/offsets.
We could enforce this as a compile-time check with something along the lines of :
#define BUILD_BUG_ON_NOT_OFFSETABLE(x) (void)(x)[0]
And use it both in rcu_assign_pointer() and rcu_dereference(). It would check for any type passed to rcu_assign_pointer and rcu_dereference which is not either a pointer or an array.
Then if someone really want to shoot himself in the foot by casting a pointer to a long after the rcu_deref, that's his problem.
Hrm, looking at rcu_assign_pointer tells me that the ((v) != NULL) test should probably already complain if v is not a pointer. So my build test is probably unneeded.
Mathieu
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
| |