linux-kernel - Re: smp_call_function_single with wait=0 considered harmful

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Date:	Thu, 5 Dec 2013 14:43:03 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tony Luck <tony.luck@...el.com>,
	Robert Richter <rric@...nel.org>,
	Aaro Koskinen <aaro.koskinen@....fi>,
	David Daney <david.daney@...ium.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: smp_call_function_single with wait=0 considered harmful

On Wed, Dec 4, 2013 at 9:46 AM, Christoph Hellwig <hch@...radead.org> wrote:
> While doing my recent work on the generic smp function calls I noticed
> that smp_call_function_single without the wait flag can't work, as
> it allocates struct call_single_data on stack, and without the wait
> flag will happily return before the IPI has been executed.

I don't understand the problem yet.  With wait==0,
smp_call_function_single() sets "csd = &__get_cpu_var(csd_data)", so
it's not using a struct on the stack.  We'll queue up "func" and
likely will return before it is executed, but that should be fine
because nobody will overwrite csd_data until it *is* executed and
csd_unlock() has been called.

> This affects the following callers:
>
>   arch/ia64/kernel/mca.c:mca_cpu_callback()
>   arch/ia64/kernel/smpboot.c:ia64_sync_itc()
>   arch/x86/kernel/kvm.c:kvm_cpu_notify()
>   arch/x86/oprofile/nmi_int.c:oprofile_cpu_notifier()
>   arch/x86/pci/amd_bus.c:amd_cpu_notify()

I don't see any reason why amd_cpu_notify() needs to use wait==0.

>   drivers/staging/octeon/ethernet-rx.c:cvm_oct_enable_one_cpu()
>   kernel/stop_machine.c:stop_two_cpus()
>
> It would be good to get these fixed so that we could remove the
> parameter.  Either convert them to wait, or use a preallocated
> call_single_data and __smp_call_function_single.
>
> After that I'd like to remove the wait argument to prevent further
> abuses.
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