linux-kernel - Re: [GIT pull] locking/urgent for v5.10-rc6

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Date:   Tue, 1 Dec 2020 08:53:21 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [GIT pull] locking/urgent for v5.10-rc6

On Tue, Dec 01, 2020 at 03:55:19PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 01, 2020 at 06:46:44AM -0800, Paul E. McKenney wrote:
> 
> > > So after having talked to Sven a bit, the thing that is happening, is
> > > that this is the one place where we take interrupts with RCU being
> > > disabled. Normally RCU is watching and all is well, except during idle.
> > 
> > Isn't interrupt entry supposed to invoke rcu_irq_enter() at some point?
> > Or did this fall victim to recent optimizations?
> 
> It does, but the problem is that s390 is still using
> trace_hardirqs_off(), which calls into tracing before RCU is enabled.
> 
> The entry order between lockdep, rcu and tracing is critical.
> 
> You can't call into tracing without RCU running,
> you can't call into RCU without lockdep setup,
> you can't call the (old) lockdep setup without landing in tracing.

Whew!  ;-)

							Thanx, Paul