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Date:   Mon, 24 Apr 2023 12:29:00 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Boqun Feng <boqun.feng@gmail.com>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        Joel Fernandes <joel@joelfernandes.org>,
        Zhouyi Zhou <zhouzhouyi@gmail.com>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        rcu <rcu@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>, lance@osuosl.org,
        "Paul E. McKenney" <paulmck@kernel.org>
Subject: Re: BUG : PowerPC RCU: torture test failed with __stack_chk_fail
Message-ID: <20230424172900.GR19790@gate.crashing.org>
References: <CAABZP2xJRGhPmfB-PrfesQKzP7fsuZsj+3TewAiLLW8u=YK4dg@mail.gmail.com> <CAEXW_YSSGYgqTpxqbYikCFS9t=2f+L-0phbU+gAAngB5z-FbyA@mail.gmail.com> <ZEXOMC2casTlobE1@boqun-archlinux> <87fs8pzalj.fsf@mail.concordia> <20230424151351.GP19790@gate.crashing.org> <ZEagN1jJwg+rUzX4@boqun-archlinux>
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On Mon, Apr 24, 2023 at 08:28:55AM -0700, Boqun Feng wrote:
> On Mon, Apr 24, 2023 at 10:13:51AM -0500, Segher Boessenkool wrote:
> > At what points can r13 change?  Only when some particular functions are
> > called?
> 
> r13 is the local paca:
> 
> 	register struct paca_struct *local_paca asm("r13");
> 
> , which is a pointer to percpu data.

Yes, it is a global register variable.

> So if a task schedule from one CPU to anotehr CPU, the value gets
> changed.

But the compiler does not see that something else changes local_paca (or
r13 some other way, via assembler code perhaps)?  Or is there a compiler
bug?

If the latter is true:

Can you make a reproducer and open a GCC PR?  <https://gcc.gnu.org/bugs/>
for how to get started doing that.  We need *exact* code that shows the
problem, together with a compiler command line.  So that we can
reproduce the problem.  That is step 0 in figuring out what is going on,
and then maybe fixing the problem :-)


Segher