linux-kernel - Re: [RFC PATCH 1/1] ppc/smp: Replace unnecessary 'while' by 'if'

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Date:   Fri, 27 Mar 2020 08:40:05 +1100
From:   Paul Mackerras <paulus@...abs.org>
To:     Leonardo Bras <leonardo@...ux.ibm.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Allison Randal <allison@...utok.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] ppc/smp: Replace unnecessary 'while' by 'if'

On Thu, Mar 26, 2020 at 05:37:52PM -0300, Leonardo Bras wrote:
> spin_until_cond() will wait until nmi_ipi_busy == false, and
> nmi_ipi_lock_start() does not seem to change nmi_ipi_busy, so there is
> no way this while will ever repeat.
> 
> Replace this 'while' by an 'if', so it does not look like it can repeat.

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 == true.
At that point it needs to go through the while loop body once more.

Paul.