[lttng-dev] liburcu: LTO breaking rcu_dereference on arm64 and possibly other architectures ?

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Mon Apr 19 11:31:45 EDT 2021

----- On Apr 19, 2021, at 5:41 AM, Duncan Sands baldrick at free.fr wrote:

>> Quick question: should we use __atomic_load() or atomic_load_explicit() (C) and
>> (std::atomic<__typeof__(x)>)(x)).load() (C++) ?
> If both are available, is there any advantage to using the C++ version when
> compiling C++?  As opposed to using the C11 one for both C and C++?

I recently noticed that using C11/C++11 atomic load explicit is not a good
fit for rcu_dereference, because we want the type to be a pointer, not an
_Atomic type. gcc appears to accept a looser typing, but clang has issues
trying to build that code.

So I plan to use __atomic(p, v, __ATOMIC_CONSUME) instead in both C and C++.

Also, I'll drop the cmm_smp_read_barrier_depends() when using __ATOMIC_CONSUME,
because AFAIU their memory ordering semantics are redundant for rcu_dereference.

Here is the resulting commit for review on gerrit:

https://review.lttng.org/c/userspace-rcu/+/5455 Fix: use __atomic_load() rather than atomic load explicit [NEW]



Mathieu Desnoyers
EfficiOS Inc.

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