Re: [patch] i386/x86_64: smp_call_function locking inconsistency

["Satyam Sharma" <satyam.sharma@gmail.com>]

Hi,

I'm about six months late here(!), but I noticed this bug in
arch/x86_64/kernel/smp.c while preparing another related
patch today and then found this thread during Googling ...

On 2/9/07, Heiko Carstens <heiko.carstens@de.ibm.com> wrote:
> On i386/x86_64 smp_call_function_single() takes call_lock with
> spin_lock_bh(). To me this would imply that it is legal to call
> smp_call_function_single() from softirq context.
> It's not since smp_call_function() takes call_lock with just
> spin_lock(). We can easily deadlock:
>
> -> [process context]
> -> smp_call_function()
> -> spin_lock(&call_lock)
> -> IRQ -> do_softirq -> tasklet
> -> [softirq context]
> -> smp_call_function_single()
> -> spin_lock_bh(&call_lock)
> -> dead

You're absolutely right, and this bug still exists in the latest -git.

> So either all spin_lock_bh's should be converted to spin_lock,
> which would limit smp_call_function()/smp_call_function_single()
> to process context & irqs enabled.
> Or the spin_lock's could be converted to spin_lock_bh which would
> make it possible to call these two functions even if in softirq
> context. AFAICS this should be safe.

Actually, I agree with David and Andi here:

On 2/9/07, David Miller <davem@davemloft.net> wrote:
> I think it's logically simpler if we disallow smp_call_function*()
> from any kind of asynchronous context.  But I'm sure your driver
> has a true need for this for some reason.

and

On 2/9/07, Andi Kleen <ak@suse.de> wrote:
> I'm not so sure. Perhaps drop _bh in both and stick a WARN_ON_ONCE in
> to catch the cases?

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.

As for:

On 2/9/07, Heiko Carstens <heiko.carstens@de.ibm.com> wrote:
> Another thing that comes into my mind is smp_call_function together
> with cpu hotplug. Who is responsible that preemption and with that
> cpu hotplug is disabled?
> Is it the caller or smp_call_function itself?
> If it's smp_call_function then s390 would be broken, since
> then we would have
> int cpus = num_online_cpus()-1;
> in preemptible context... I agree: what a mess :)

and

On 2/9/07, Jan Glauber <jan.glauber@de.ibm.com> wrote:
> If preemption must be disabled before smp_call_function() we should have
> the same semantics for all smp_call_function_* variants.

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? ]

Satyam
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