[lttng-dev] liburcu: LTO breaking rcu_dereference on arm64 and possibly other architectures ?
Mathieu Desnoyers
mathieu.desnoyers at efficios.com
Fri Apr 16 16:39:38 EDT 2021
----- On Apr 16, 2021, at 11:22 AM, lttng-dev lttng-dev at lists.lttng.org wrote:
> Hi Mathieu,
>
Hi Duncan,
> On 4/16/21 4:52 PM, Mathieu Desnoyers via lttng-dev wrote:
>> Hi Paul, Will, Peter,
>>
>> I noticed in this discussion https://lkml.org/lkml/2021/4/16/118 that LTO
>> is able to break rcu_dereference. This seems to be taken care of by
>> arch/arm64/include/asm/rwonce.h on arm64 in the Linux kernel tree.
>>
>> In the liburcu user-space library, we have this comment near rcu_dereference()
>> in
>> include/urcu/static/pointer.h:
>>
>> * The compiler memory barrier in CMM_LOAD_SHARED() ensures that
>> value-speculative
>> * optimizations (e.g. VSS: Value Speculation Scheduling) does not perform the
>> * data read before the pointer read by speculating the value of the pointer.
>> * Correct ordering is ensured because the pointer is read as a volatile access.
>> * This acts as a global side-effect operation, which forbids reordering of
>> * dependent memory operations. Note that such concern about dependency-breaking
>> * optimizations will eventually be taken care of by the "memory_order_consume"
>> * addition to forthcoming C++ standard.
>>
>> (note: CMM_LOAD_SHARED() is the equivalent of READ_ONCE(), but was introduced in
>> liburcu as a public API before READ_ONCE() existed in the Linux kernel)
>
> this is not directly on topic, but what do you think of porting userspace RCU to
> use the C++ memory model and GCC/LLVM atomic builtins (__atomic_store etc)
> rather than rolling your own? Tools like thread sanitizer would then understand
> what userspace RCU is doing. Not to mention the compiler. More developers
> would understand it too!
Yes, that sounds like a clear win.
> From a code organization viewpoint, going down this path would presumably mean
> directly using GCC/LLVM atomic support when available, and falling back on
> something like the current uatomic to emulate them for older compilers.
Yes, I think this approach would be good. One caveat though: the GCC atomic
operations were known to be broken with some older compilers for specific architectures,
so we may have to keep track of a list of known buggy compilers to use our own
implementation instead in those situations. It's been a while since I've looked at
this though, so we may not even be supporting those old compilers in liburcu anymore.
>
> Some parts of uatomic have pretty clear equivalents (see below), but not all, so
> the conversion could be quite tricky.
We'd have to see on a case by case basis, but it cannot hurt to start the effort
by integrating the easy ones.
>
>> Peter tells me the "memory_order_consume" is not something which can be used
>> today.
>
> This is a pity, because it seems to have been invented with rcu_dereference in
> mind.
Actually, (see other leg of this email thread) memory_order_consume works for
rcu_dereference, but it appears to be implemented as a slightly heavier than
required memory_order_acquire on weakly-ordered architectures. So we're just
moving the issue into compiler-land. Oh well.
>
>> Any information on its status at C/C++ standard levels and implementation-wise ?
>>
>> Pragmatically speaking, what should we change in liburcu to ensure we don't
>> generate
>> broken code when LTO is enabled ? I suspect there are a few options here:
>>
>> 1) Fail to build if LTO is enabled,
>> 2) Generate slower code for rcu_dereference, either on all architectures or only
>> on weakly-ordered architectures,
>> 3) Generate different code depending on whether LTO is enabled or not. AFAIU
>> this would only
>> work if every compile unit is aware that it will end up being optimized with
>> LTO. Not sure
>> how this could be done in the context of user-space.
>> 4) [ Insert better idea here. ]
>>
>> Thoughts ?
>
> Best wishes, Duncan.
>
> PS: We are experimentally running with the following patch, as it already makes
> thread sanitizer a lot happier:
Quick question: should we use __atomic_load() or atomic_load_explicit() (C) and
(std::atomic<__typeof__(x)>)(x)).load() (C++) ?
We'd have to make this dependent on C11/C++11 though, and keep volatile for older
compilers.
Last thing: I have limited time to work on this, so if you have well-tested patches
you wish to submit, I'll do my best to review them!
Thanks,
Mathieu
>
> --- a/External/UserspaceRCU/userspace-rcu/include/urcu/system.h
>
> +++ b/External/UserspaceRCU/userspace-rcu/include/urcu/system.h
>
> @@ -26,34 +26,45 @@
>
> * Identify a shared load. A cmm_smp_rmc() or cmm_smp_mc() should come
>
> * before the load.
>
> */
>
> -#define _CMM_LOAD_SHARED(p) CMM_ACCESS_ONCE(p)
>
> +#define _CMM_LOAD_SHARED(p) \
>
> + __extension__ \
>
> + ({ \
>
> + __typeof__(p) v; \
>
> + __atomic_load(&p, &v, __ATOMIC_RELAXED); \
>
> + v; \
>
> + })
>
>
>
> /*
>
> * Load a data from shared memory, doing a cache flush if required.
>
> */
>
> -#define CMM_LOAD_SHARED(p) \
>
> - __extension__ \
>
> - ({ \
>
> - cmm_smp_rmc(); \
>
> - _CMM_LOAD_SHARED(p); \
>
> +#define CMM_LOAD_SHARED(p) \
>
> + __extension__ \
>
> + ({ \
>
> + __typeof__(p) v; \
>
> + __atomic_load(&p, &v, __ATOMIC_ACQUIRE); \
>
> + v; \
>
> })
>
>
>
> /*
>
> * Identify a shared store. A cmm_smp_wmc() or cmm_smp_mc() should
>
> * follow the store.
>
> */
>
> -#define _CMM_STORE_SHARED(x, v) __extension__ ({ CMM_ACCESS_ONCE(x) = (v); })
>
> +#define _CMM_STORE_SHARED(x, v) \
>
> + __extension__ \
>
> + ({ \
>
> + __typeof__(x) w = v; \
>
> + __atomic_store(&x, &w, __ATOMIC_RELAXED); \
>
> + })
>
>
>
> /*
>
> * Store v into x, where x is located in shared memory. Performs the
>
> * required cache flush after writing. Returns v.
>
> */
>
> -#define CMM_STORE_SHARED(x, v) \
>
> - __extension__ \
>
> - ({ \
>
> - __typeof__(x) _v = _CMM_STORE_SHARED(x, v); \
>
> - cmm_smp_wmc(); \
>
> - _v = _v; /* Work around clang "unused result" */ \
>
> +#define CMM_STORE_SHARED(x, v) \
>
> + __extension__ \
>
> + ({ \
>
> + __typeof__(x) w = v; \
>
> + __atomic_store(&x, &w, __ATOMIC_RELEASE); \
>
> })
>
>
>
> #endif /* _URCU_SYSTEM_H */
>
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--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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