On Thu, Jan 24, 2019 at 07:01:09PM +0100, Peter Zijlstra wrote: > > Thanks for having kernel/locking people on Cc... > > On Wed, Jan 23, 2019 at 08:13:55PM -0800, Alexei Starovoitov wrote: > > > Implementation details: > > - on !SMP bpf_spin_lock() becomes nop > > Because no BPF program is preemptible? I don't see any assertions or > even a comment that says this code is non-preemptible. > > AFAICT some of the BPF_RUN_PROG things are under rcu_read_lock() only, > which is not sufficient. > > > - on architectures that don't support queued_spin_lock trivial lock is used. > > Note that arch_spin_lock cannot be used, since not all archs agree that > > zero == unlocked and sizeof(arch_spinlock_t) != sizeof(__u32). > > I really don't much like direct usage of qspinlock; esp. not as a > surprise. > > Why does it matter if 0 means unlocked; that's what > __ARCH_SPIN_LOCK_UNLOCKED is for. > > I get the sizeof(__u32) thing, but why not key off of that? > > > Next steps: > > - allow bpf_spin_lock in other map types (like cgroup local storage) > > - introduce BPF_F_LOCK flag for bpf_map_update() syscall and helper > > to request kernel to grab bpf_spin_lock before rewriting the value. > > That will serialize access to map elements. > > 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? >
Also; what about BPF running from NMI context and using locks?
