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SubjectRe: [patch 01/15] Kernel Tracepoints
On Tue, Jul 15, 2008 at 12:08:13PM -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 big question is "why"? smp_read_barrier_depends() is pretty
lightweight, after all.

Thanx, Paul

> 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 ?
>
> Mathieu
>
> > > > If so, does this really matter since we're already in an unlikely
> > > > section? Again, if so, this deserves a comment ;-)
> > > >
> > > > [ still think those preempt_* calls should be called
> > > > rcu_read_sched_lock() or such. ]
> > > >
> > > > Anyway, does this still generate better code?
> > > >
> > >
> > > On x86_64 :
> > >
> > > 820: bf 01 00 00 00 mov $0x1,%edi
> > > 825: e8 00 00 00 00 callq 82a <thread_return+0x136>
> > > 82a: 48 8b 1d 00 00 00 00 mov 0x0(%rip),%rbx # 831 <thread_return+0x13d>
> > > 831: 48 85 db test %rbx,%rbx
> > > 834: 75 21 jne 857 <thread_return+0x163>
> > > 836: eb 27 jmp 85f <thread_return+0x16b>
> > > 838: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1)
> > > 83f: 00
> > > 840: 48 8b 95 68 ff ff ff mov -0x98(%rbp),%rdx
> > > 847: 48 8b b5 60 ff ff ff mov -0xa0(%rbp),%rsi
> > > 84e: 4c 89 e7 mov %r12,%rdi
> > > 851: 48 83 c3 08 add $0x8,%rbx
> > > 855: ff d0 callq *%rax
> > > 857: 48 8b 03 mov (%rbx),%rax
> > > 85a: 48 85 c0 test %rax,%rax
> > > 85d: 75 e1 jne 840 <thread_return+0x14c>
> > > 85f: bf 01 00 00 00 mov $0x1,%edi
> > > 864:
> > >
> > > for 68 bytes.
> > >
> > > My original implementation was 77 bytes, so yes, we have a win.
> >
> > Ah, good good ! :-)
> >
>
> --
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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