From: "Paul E. McKenney via lttng-dev" <lttng-dev@lists.lttng.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Will Deacon <will@kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
lttng-dev <lttng-dev@lists.lttng.org>,
Carlos O'Donell <carlos@redhat.com>
Subject: Re: [lttng-dev] liburcu: LTO breaking rcu_dereference on arm64 and possibly other architectures ?
Date: Fri, 16 Apr 2021 13:01:46 -0700 [thread overview]
Message-ID: <20210416200146.GK4212@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <2056094038.84390.1618601453585.JavaMail.zimbra@efficios.com>
On Fri, Apr 16, 2021 at 03:30:53PM -0400, Mathieu Desnoyers wrote:
> ----- On Apr 16, 2021, at 3:02 PM, paulmck paulmck@kernel.org wrote:
> [...]
> >
> > If it can be done reasonably, I suggest also having some way for the
> > person building userspace RCU to say "I know what I am doing, so do
> > it with volatile rather than memory_order_consume."
>
> Like so ?
>
> #define CMM_ACCESS_ONCE(x) (*(__volatile__ __typeof__(x) *)&(x))
> #define CMM_LOAD_SHARED(p) CMM_ACCESS_ONCE(p)
>
> /*
> * By defining URCU_DEREFERENCE_USE_VOLATILE, the user requires use of
> * volatile access to implement rcu_dereference rather than
> * memory_order_consume load from the C11/C++11 standards.
> *
> * This may improve performance on weakly-ordered architectures where
> * the compiler implements memory_order_consume as a
> * memory_order_acquire, which is stricter than required by the
> * standard.
> *
> * Note that using volatile accesses for rcu_dereference may cause
> * LTO to generate incorrectly ordered code starting from C11/C++11.
> */
>
> #ifdef URCU_DEREFERENCE_USE_VOLATILE
> # define rcu_dereference(x) CMM_LOAD_SHARED(x)
> #else
> # 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
> #endif
Looks good to me!
Thanx, Paul
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next prev parent reply other threads:[~2021-04-16 20:01 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-16 14:52 [lttng-dev] liburcu: LTO breaking rcu_dereference on arm64 and possibly other architectures ? Mathieu Desnoyers via lttng-dev
2021-04-16 15:17 ` Peter Zijlstra via lttng-dev
2021-04-16 16:01 ` Paul E. McKenney via lttng-dev
2021-04-16 18:40 ` Mathieu Desnoyers via lttng-dev
2021-04-16 19:02 ` Paul E. McKenney via lttng-dev
2021-04-16 19:30 ` Mathieu Desnoyers via lttng-dev
2021-04-16 20:01 ` Paul E. McKenney via lttng-dev [this message]
2021-04-16 15:22 ` Duncan Sands via lttng-dev
2021-04-16 20:39 ` Mathieu Desnoyers via lttng-dev
[not found] ` <7972b031-59b9-7fb5-6379-58bcec13a769@free.fr>
2021-04-19 15:31 ` Mathieu Desnoyers via lttng-dev
2021-04-19 15:41 ` Duncan Sands via lttng-dev
2021-04-19 15:54 ` Mathieu Desnoyers via lttng-dev
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