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SubjectRe: RCU stall in af_unix.c, should use spin_lock_irqsave?
Dear Eric Dumazet,

On Tue, 21 Oct 2014 03:28:20 -0700, Eric Dumazet wrote:

> > Ok. So it's actually safe to mix spin_lock() and spin_lock_irqsave() on
> > the same lock, if you know that this lock will never ever be taken in
> > an interrupt context?
> Sure.

Ok, thanks.

> > > mvpp2 is seriously brain damaged : on_each_cpu() cannot be used from
> > > a bottom half handler.
> >
> > That's what I thought. Back to the drawing board then, to fix mvpp2.
> >
> > Do you think there is a place where we can write down those
> > assumptions? It isn't easy to spot whether on_each_cpu() is safe to use
> > in a bottom half or not.
> >
> Really ? kernel/smp.c is full of comments.
> Too many comments and people seem to not read them ;)
> Take a look at smp_call_function(), which is called from on_each_cpu()

Indeed, it's written black on white on smp_call_function(). I guess
we'll have to dig into the details of the mvpp2 hardware and its
per-CPU registers and see how to handle things properly.

Thanks a lot for your input!

Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering

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