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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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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