linux-kernel - Re: BUG : PowerPC RCU: torture test failed with __stack_chk

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Message-ID: <c8f9896e-0614-7aff-3846-80d2c1731209@csgroup.eu>
Date:   Fri, 28 Apr 2023 10:35:01 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Joel Fernandes <joel@...lfernandes.org>,
        Zhouyi Zhou <zhouzhouyi@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        rcu <rcu@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "lance@...osl.org" <lance@...osl.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: BUG : PowerPC RCU: torture test failed with __stack_chk_fail



Le 26/04/2023 à 14:29, Michael Ellerman a écrit :
> Joel Fernandes <joel@...lfernandes.org> writes:
>> On Tue, Apr 25, 2023 at 6:58 AM Zhouyi Zhou <zhouzhouyi@...il.com> wrote:
> ...
>>
>> Out of curiosity for PPC folks, why cannot 64-bit PPC use per-task
>> canary? Michael, is this an optimization? Adding Christophe as well
>> since it came in a few years ago via the following commit:
> 
> I think Christophe also answered these in his reply.
> 
> We do use a per-task canary, but because we don't have "current" in a
> register, we can't use the value in current for GCC.
> 
> In one of my replies I said a possible solution would be to keep current
> in a register on 64-bit, but we'd need to do that in addition to the
> paca, so that would consume another GPR which we'd need to think hard
> about.

An analysis was done here https://github.com/linuxppc/issues/issues/45 
showing that r14 is very little used.

> 
> There's another reason to have it in the paca, which is that the paca is
> always accessible, even when the MMU is off, whereas current isn't (in
> some situations).

Even now that powerpc is converted to CONFIG_THREAD_INFO_IN_TASK ?

Christophe