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Message-ID: <f86aa1672b447bd09a214bc8682a70934dcee82f.camel@linux.ibm.com>
Subject: Re: [RFC PATCH 1/1] ppc/smp: Replace unnecessary 'while' by 'if'
From:   Leonardo Bras <leonardo@linux.ibm.com>
To:     Paul Mackerras <paulus@ozlabs.org>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Allison Randal <allison@lohutok.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Date:   Thu, 26 Mar 2020 19:05:03 -0300
In-Reply-To: <20200326214005.GB9894@blackberry>
References: <20200326203752.497029-1-leonardo@linux.ibm.com>
         <20200326214005.GB9894@blackberry>
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On Fri, 2020-03-27 at 08:40 +1100, Paul Mackerras wrote:
> On Thu, Mar 26, 2020 at 05:37:52PM -0300, Leonardo Bras wrote:
> > spin_until_cond() will wait until nmi_ipi_busy =3D=3D false, and
> > nmi_ipi_lock_start() does not seem to change nmi_ipi_busy, so there is
> > no way this while will ever repeat.
> >=20
> > Replace this 'while' by an 'if', so it does not look like it can repeat=
.
>=20
> Nack, it can repeat.  The scenario is that cpu A is in this code,
> inside spin_until_cond(); cpu B has previously set nmi_ipi_busy, and
> cpu C is also waiting for nmi_ipi_busy to be cleared, like cpu A.
> When cpu B clears nmi_ipi_busy, both cpu A and cpu C will see that and
> will race inside nmi_ipi_lock_start().  One of them, say cpu C, will
> take the lock and proceed to set nmi_ipi_busy and then call
> nmi_ipi_unlock().  Then the other cpu (cpu A) will then take the lock
> and return from nmi_ipi_lock_start() and find nmi_ipi_busy =3D=3D true.
> At that point it needs to go through the while loop body once more.
>=20
> Paul.

Ok, got it.

Thanks for explaining Paul!

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