From: Peter Zijlstra <peterz@infradead.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: paulmck@kernel.org, rcu@vger.kernel.org,
linux-kernel@vger.kernel.org, kernel-team@fb.com,
mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
oleg@redhat.com
Subject: Re: [PATCH tip/core/rcu 1/4] srcu: Fix __call_srcu()/process_srcu() datarace
Date: Mon, 17 Feb 2020 18:11:04 +0100 [thread overview]
Message-ID: <20200217171104.GV14914@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200217170157.GA166797@google.com>
On Mon, Feb 17, 2020 at 12:01:57PM -0500, Joel Fernandes wrote:
> Peter it sounds like you have a failure scenario in mind. Could you describe
> more if so?
>
> I am curious if you were thinking of invented-stores issue here.
>
> For educational purposes, I was trying to come up with an example where my
> compiler does something bad to code without WRITE_ONCE(). So far I only can
> reproduce a write-tearing example when write with an immediate value is split
> into 2 writes, like Will mentioned:
> http://lore.kernel.org/r/20190821103200.kpufwtviqhpbuv2n@willie-the-truck
> But that does not seem to apply to this code.
> > > - snp->srcu_gp_seq_needed_exp = gpseq;
> > > + WRITE_ONCE(snp->srcu_gp_seq_needed_exp, gpseq);
Yeah, store tearing. No sane compiler will actually do that, but it is
allowed to do random permutations of byte stores just to fuck with us.
WRITE_ONCE() disallows that.
In that case, the READ_ONCE()s could observe garbage and the compare
might accidentally report the wrong thing.
next prev parent reply other threads:[~2020-02-17 17:11 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-15 0:29 [PATCH tip/core/rcu 0/4] SRCU updates for v5.7 Paul E. McKenney
2020-02-15 0:29 ` [PATCH tip/core/rcu 1/4] srcu: Fix __call_srcu()/process_srcu() datarace paulmck
2020-02-17 12:42 ` Peter Zijlstra
2020-02-17 17:01 ` Joel Fernandes
2020-02-17 17:11 ` Peter Zijlstra [this message]
2020-02-17 17:46 ` Joel Fernandes
2020-02-17 18:18 ` Paul E. McKenney
2020-02-15 0:29 ` [PATCH tip/core/rcu 2/4] srcu: Fix __call_srcu()/srcu_get_delay() datarace paulmck
2020-02-15 0:29 ` [PATCH tip/core/rcu 3/4] srcu: Fix process_srcu()/srcu_batches_completed() datarace paulmck
2020-02-15 0:29 ` [PATCH tip/core/rcu 4/4] srcu: Add READ_ONCE() to srcu_struct ->srcu_gp_seq load paulmck
2020-02-17 12:45 ` Peter Zijlstra
2020-02-17 18:32 ` Paul E. McKenney
2020-02-17 23:06 ` Paul E. McKenney
2020-02-18 11:46 ` Peter Zijlstra
2020-02-18 21:48 ` Paul E. McKenney
2020-02-18 11:43 ` Peter Zijlstra
2020-02-18 16:34 ` Paul E. McKenney
2020-02-18 20:26 ` Peter Zijlstra
2020-02-18 21:38 ` Paul E. McKenney
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