From: Paul Mackerras <paulus@ozlabs.org>
To: Leonardo Bras <leonardo@linux.ibm.com>
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
Subject: Re: [RFC PATCH 1/1] ppc/smp: Replace unnecessary 'while' by 'if'
Date: Fri, 27 Mar 2020 08:40:05 +1100 [thread overview]
Message-ID: <20200326214005.GB9894@blackberry> (raw)
In-Reply-To: <20200326203752.497029-1-leonardo@linux.ibm.com>
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.
WARNING: multiple messages have this Message-ID (diff)
From: Paul Mackerras <paulus@ozlabs.org>
To: Leonardo Bras <leonardo@linux.ibm.com>
Cc: linux-kernel@vger.kernel.org,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
linuxppc-dev@lists.ozlabs.org,
Allison Randal <allison@lohutok.net>
Subject: Re: [RFC PATCH 1/1] ppc/smp: Replace unnecessary 'while' by 'if'
Date: Fri, 27 Mar 2020 08:40:05 +1100 [thread overview]
Message-ID: <20200326214005.GB9894@blackberry> (raw)
In-Reply-To: <20200326203752.497029-1-leonardo@linux.ibm.com>
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.
next prev parent reply other threads:[~2020-03-26 21:58 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-26 20:37 [RFC PATCH 1/1] ppc/smp: Replace unnecessary 'while' by 'if' Leonardo Bras
2020-03-26 21:40 ` Paul Mackerras [this message]
2020-03-26 21:40 ` Paul Mackerras
2020-03-26 22:05 ` Leonardo Bras
2020-03-26 22:05 ` Leonardo Bras
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