On Thu, Jan 24, 2019 at 03:58:59PM -0800, Alexei Starovoitov wrote:
> On Thu, Jan 24, 2019 at 07:01:09PM +0100, Peter Zijlstra wrote:
> > So clearly this map stuff is shared between bpf proglets, otherwise
> > there would not be a need for locking. But what happens if one is from
> > task context and another from IRQ context?
> >
> > I don't see a local_irq_save()/restore() anywhere. What avoids the
> > trivial lock inversion?
>
> > and from NMI ...
>
> progs are not preemptable and map syscall accessors have bpf_prog_active
> counters.
> So nmi/kprobe progs will not be running when syscall is running.
> Hence dead lock is not possible and irq_save is not needed.
What about the progs that run from SoftIRQ ? Since that bpf_prog_active
thing isn't inside BPF_PROG_RUN() what is to stop say:
reuseport_select_sock()
...
BPF_PROG_RUN()
bpf_spin_lock()
<IRQ>
...
BPF_PROG_RUN()
bpf_spin_lock() // forever more
</IRQ>
Unless you stick that bpf_prog_active stuff inside BPF_PROG_RUN itself,
I don't see how you can fundamentally avoid this happening (now or in
the future).
