linux-kernel - Re: liburcu: LTO breaking rcu_dereference on arm64 and possibly other architectures ?

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Message-ID: <2056094038.84390.1618601453585.JavaMail.zimbra@efficios.com>
Date:   Fri, 16 Apr 2021 15:30:53 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     paulmck <paulmck@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        lttng-dev <lttng-dev@...ts.lttng.org>,
        Carlos O'Donell <carlos@...hat.com>
Subject: Re: liburcu: LTO breaking rcu_dereference on arm64 and possibly
 other architectures ?

----- On Apr 16, 2021, at 3:02 PM, paulmck paulmck@...nel.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

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com