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    SubjectRe: [patch 01/15] Kernel Tracepoints
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    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.





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