linux-kernel - Re: [patch] i386/x86_64: smp_call

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Date:	Thu, 7 Jun 2007 22:24:01 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	"Heiko Carstens" <heiko.carstens@...ibm.com>
Cc:	"Jan Glauber" <jan.glauber@...ibm.com>,
	"David Miller" <davem@...emloft.net>, akpm@...l.org, mingo@...e.hu,
	ak@...e.de, schwidefsky@...ibm.com, linux-kernel@...r.kernel.org,
	"Alan Cox" <alan@...hat.com>
Subject: Re: [patch] i386/x86_64: smp_call_function locking inconsistency

Hi Heiko,

On 6/7/07, Heiko Carstens <heiko.carstens@...ibm.com> wrote:
> > Replacing the _bh variants and making smp_call_function{_single}
> > illegal from all contexts but process is fine for x86_64, as we
> > don't really have any driver that needs to use this from softirq
> > context in the x86_64 tree. This means it becomes dissimilar to
> > s390, but similar to powerpc, mips, alpha, sparc64 semantics.
> > I'll prepare and submit a patch for the same, shortly.
>
> Calling an smp_call_* function from any context but process context is
> a bug. We didn't notice this initially when we used smp_call_function
> from softirq context... until we deadlocked ;)
> So s390 is the same as any other architecture wrt this.

I'll fix the necessary patch for x86_64, in that case.

> > I don't see any CPU hotplug / preemption disabling issues here.
> > Note that both smp_call_function() and smp_call_function_single()
> > on x86_64 acquire the call_lock spinlock before using cpu_online_map
> > via num_online_cpus(). And spin_lock() does preempt_disable() on both
> > SMP and !SMP, so we're safe. [ But we're not explicitly disabling
> > preemption and depending on spin_lock() instead, so a comment would
> > be in order? ]
>
> Calling smp_call_function_single() with preemption enabled is pointless.
> You might be scheduled on the cpu you want to send an IPI to and get
> -EBUSY as return... If cpu hotplug is enabled the target cpu might even
> be gone when smp_call_function_single() gets executed.

Exactly. I was only mentioning that the smp_call_function*
of x86{_64} were safe anyway (but the smp_processor_id()
that would've preceded it need not have been, of course).

> Avi Kivity has already a patch which introduces an on_cpu() function which
> looks quite like on_each_cpu(). That way you don't have to open code this
> stuff over and over again:
>
> preempt_disable();
> if (cpu == smp_processor_id())
>         func();
> else
>         smp_call_function_single(...);
> preempt_enable();
>
> There are already quite a few of these around.

Indeed -- this was doubly problematic because the un-safeness
was because of smp_processor_id() as well as the (eventual)
access of cpu_online_map (via smp_call_function() ->
num_online_cpus()) ... thanks for letting me know about this.

Satyam
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