linux-kernel - Re: [PATCH] rcutorture: Fix rcu_torture_pipe_update_one()/rcu_torture

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Date: Wed, 6 Mar 2024 22:06:21 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>, paulmck@...nel.org
Cc: Steven Rostedt <rostedt@...dmis.org>, linke li <lilinke99@...com>,
 joel@...lfernandes.org, boqun.feng@...il.com, dave@...olabs.net,
 frederic@...nel.org, jiangshanlai@...il.com, josh@...htriplett.org,
 linux-kernel@...r.kernel.org, qiang.zhang1211@...il.com,
 quic_neeraju@...cinc.com, rcu@...r.kernel.org
Subject: Re: [PATCH] rcutorture: Fix
 rcu_torture_pipe_update_one()/rcu_torture_writer() data race and concurrency
 bug

On 2024-03-06 21:43, Linus Torvalds wrote:
[...]
> 
> Honestly, this all makes me think that we'd be *much* better off
> showing the real "handoff" with smp_store_release() and
> smp_load_acquire().

We've done something similar in liburcu (userspace code) to allow
Thread Sanitizer to understand the happens-before relationships
within the RCU implementations and lock-free data structures.

Moving to load-acquire/store-release (C11 model in our case)
allowed us to provide enough happens-before relationship for
Thread Sanitizer to understand what is happening under the
hood in liburcu and perform relevant race detection of user
code.

As far as the WRITE_ONCE(x, READ_ONCE(x) + 1) pattern
is concerned, the only valid use-case I can think of is
split counters or RCU implementations where there is a
single updater doing the increment, and one or more
concurrent reader threads that need to snapshot a
consistent value with READ_ONCE().

Thanks,

Mathieu

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