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Date:   Wed, 22 Sep 2021 15:02:32 +0200
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        rcu@vger.kernel.org, linux-rt-users@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Steven Price <steven.price@arm.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Mike Galbraith <efault@gmx.de>
Subject: Re: rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT
Message-ID: <20210922130232.vm7rgkdszfhejf34@linutronix.de>
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On 2021-09-22 13:38:20 [+0200], Frederic Weisbecker wrote:
> > The part with rcutree.use_softirq=0 on RT does not make it any better,
> > right?
> 
> The rcuc kthread disables softirqs before calling rcu_core(), so it behaves
> pretty much the same as a softirq. Or am I missing something?

Oh, no you don't.

> > So you rely on some implicit behaviour which breaks with RT such as:
> > 
> >                               CPU 0
> >            -----------------------------------------------
> >            RANDOM TASK-A                      RANDOM TASK-B
> >            ------                             -----------
> >            int *X = &per_cpu(CPUX, 0)         int *X = &per_cpu(CPUX, 0)
> >            int A, B;                          
> > 					      spin_lock(&D);
> >            spin_lock(&C);
> > 	   				      WRITE_ONCE(*X, 0);
> >            A = READ_ONCE(*X);
> >                                               WRITE_ONCE(*X, 1);
> >            B = READ_ONCE(*X);
> > 
> > while spinlock C and D are just random locks not related to CPUX but it
> > just happens that they are held at that time. So for !RT you guarantee
> > that A == B while it is not the case on RT.
> 
> Not sure which spinlocks you are referring to here. Also most RCU spinlocks
> are raw.

I was bringing an example where you also could rely on implicit locking
provided by spin_lock() which breaks on RT.

Sebastian