On Thu, Jan 24, 2019 at 03:42:32PM -0800, Paul E. McKenney wrote: > On Thu, Jan 24, 2019 at 07:56:52PM +0100, Peter Zijlstra wrote: > > 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. > > Substituting the lightweight-reader SRCU as discussed earlier would allow > use of a more generic locking primitive, for example, one that allowed > blocking, at least in cases were the context allowed this. > > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git > branch srcu-lr.2019.01.16a. > > One advantage of a more generic locking primitive would be keeping BPF > programs independent of internal changes to spinlock primitives.
Let's keep "srcu in bpf" discussion separate from bpf_spin_lock discussion. bpf is not switching to srcu any time soon. If/when it happens it will be only for certain prog+map types like bpf syscall probes that need to be able to do copy_from_user from bpf prog.
