----- On Apr 16, 2021, at 11:22 AM, lttng-dev lttng-dev@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 */ > > _______________________________________________ > lttng-dev mailing list > lttng-dev@lists.lttng.org > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com _______________________________________________ lttng-dev mailing list lttng-dev@lists.lttng.org https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
