----- On Apr 16, 2021, at 12:01 PM, paulmck paul...@kernel.org wrote: > On Fri, Apr 16, 2021 at 05:17:11PM +0200, Peter Zijlstra wrote: >> On Fri, Apr 16, 2021 at 10:52:16AM -0400, Mathieu Desnoyers 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) >> > >> > Peter tells me the "memory_order_consume" is not something which can be >> > used >> > today. >> > Any information on its status at C/C++ standard levels and >> > implementation-wise ? > > Actually, you really can use memory_order_consume. All current > implementations will compile it as if it was memory_order_acquire. > This will work correctly, but may be slower than you would like on ARM, > PowerPC, and so on. > > On things like x86, the penalty is forgone optimizations, so less > of a problem there.
OK > >> > 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. ] > > Use memory_order_consume if LTO is enabled. That will work now, and > might generate good code in some hoped-for future. In the context of a user-space library, how does one check whether LTO is enabled with preprocessor directives ? A quick test with gcc seems to show that both with and without -flto cannot be distinguished from a preprocessor POV, e.g. the output of both gcc --std=c11 -O2 -dM -E - < /dev/null and gcc --std=c11 -O2 -flto -dM -E - < /dev/null is exactly the same. Am I missing something here ? If we accept to use memory_order_consume all the time in both C and C++ code starting from C11 and C++11, the following code snippet could do the trick: #define CMM_ACCESS_ONCE(x) (*(__volatile__ __typeof__(x) *)&(x)) #define CMM_LOAD_SHARED(p) CMM_ACCESS_ONCE(p) #if defined (__cplusplus) # if __cplusplus >= 201103L # include <atomic> # define rcu_dereference(x) ((std::atomic<__typeof__(x)>)(x)).load(std::memory_order_consume) # else # define rcu_dereference(x) CMM_LOAD_SHARED(x) # endif #else # if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) # include <stdatomic.h> # define rcu_dereference(x) atomic_load_explicit(&(x), memory_order_consume) # else # define rcu_dereference(x) CMM_LOAD_SHARED(x) # endif #endif This uses the volatile approach prior to C11/C++11, and moves to memory_order_consume afterwards. This will bring a performance penalty on weakly-ordered architectures even when -flto is not specified though. Then the burden is pushed on the compiler people to eventually implement an efficient memory_order_consume. Is that acceptable ? Thanks, Mathieu -- 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
