Messages in this thread | | | Date | Mon, 9 Jul 2018 15:19:55 -0700 | From | Alexei Starovoitov <> | Subject | Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command |
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On Mon, Jul 09, 2018 at 03:19:03PM -0700, Paul E. McKenney wrote: > On Mon, Jul 09, 2018 at 05:35:34PM -0400, Mathieu Desnoyers wrote: > > > > > > ----- On Jul 9, 2018, at 5:09 PM, Alexei Starovoitov alexei.starovoitov@gmail.com wrote: > > > > > On Sun, Jul 08, 2018 at 04:54:38PM -0400, Mathieu Desnoyers wrote: > > >> ----- On Jul 7, 2018, at 4:33 PM, Joel Fernandes joelaf@google.com wrote: > > >> > > >> > On Fri, Jul 06, 2018 at 07:54:28PM -0700, Alexei Starovoitov wrote: > > >> >> On Fri, Jul 06, 2018 at 06:56:16PM -0700, Daniel Colascione wrote: > > >> >> > BPF_SYNCHRONIZE waits for any BPF programs active at the time of > > >> >> > BPF_SYNCHRONIZE to complete, allowing userspace to ensure atomicity of > > >> >> > RCU data structure operations with respect to active programs. For > > >> >> > example, userspace can update a map->map entry to point to a new map, > > >> >> > use BPF_SYNCHRONIZE to wait for any BPF programs using the old map to > > >> >> > complete, and then drain the old map without fear that BPF programs > > >> >> > may still be updating it. > > >> >> > > > >> >> > Signed-off-by: Daniel Colascione <dancol@google.com> > > >> >> > --- > > >> >> > include/uapi/linux/bpf.h | 1 + > > >> >> > kernel/bpf/syscall.c | 14 ++++++++++++++ > > >> >> > 2 files changed, 15 insertions(+) > > >> >> > > > >> >> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h > > >> >> > index b7db3261c62d..4365c50e8055 100644 > > >> >> > --- a/include/uapi/linux/bpf.h > > >> >> > +++ b/include/uapi/linux/bpf.h > > >> >> > @@ -98,6 +98,7 @@ enum bpf_cmd { > > >> >> > BPF_BTF_LOAD, > > >> >> > BPF_BTF_GET_FD_BY_ID, > > >> >> > BPF_TASK_FD_QUERY, > > >> >> > + BPF_SYNCHRONIZE, > > >> >> > }; > > >> >> > > > >> >> > enum bpf_map_type { > > >> >> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c > > >> >> > index d10ecd78105f..60ec7811846e 100644 > > >> >> > --- a/kernel/bpf/syscall.c > > >> >> > +++ b/kernel/bpf/syscall.c > > >> >> > @@ -2272,6 +2272,20 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, > > >> >> > uattr, unsigned int, siz > > >> >> > if (sysctl_unprivileged_bpf_disabled && !capable(CAP_SYS_ADMIN)) > > >> >> > return -EPERM; > > >> >> > > > >> >> > + if (cmd == BPF_SYNCHRONIZE) { > > >> >> > + if (uattr != NULL || size != 0) > > >> >> > + return -EINVAL; > > >> >> > + err = security_bpf(cmd, NULL, 0); > > >> >> > + if (err < 0) > > >> >> > + return err; > > >> >> > + /* BPF programs are run with preempt disabled, so > > >> >> > + * synchronize_sched is sufficient even with > > >> >> > + * RCU_PREEMPT. > > >> >> > + */ > > >> >> > + synchronize_sched(); > > >> >> > + return 0; > > >> >> > > >> >> I don't think it's necessary. sys_membarrier() can do this already > > >> >> and some folks use it exactly for this use case. > > >> > > > >> > Alexei, the use of sys_membarrier for this purpose seems kind of weird to me > > >> > though. No where does the manpage say membarrier should be implemented this > > >> > way so what happens if the implementation changes? > > >> > > > >> > Further, membarrier manpage says that a memory barrier should be matched with > > >> > a matching barrier. In this use case there is no matching barrier, so it > > >> > makes it weirder. > > >> > > > >> > Lastly, sys_membarrier seems will not work on nohz-full systems, so its a bit > > >> > fragile to depend on it for this? > > >> > > > >> > case MEMBARRIER_CMD_GLOBAL: > > >> > /* MEMBARRIER_CMD_GLOBAL is not compatible with nohz_full. */ > > >> > if (tick_nohz_full_enabled()) > > >> > return -EINVAL; > > >> > if (num_online_cpus() > 1) > > >> > synchronize_sched(); > > >> > return 0; > > >> > > > >> > > > >> > Adding Mathieu as well who I believe is author/maintainer of membarrier. > > >> > > >> See commit 907565337 > > >> "Fix: Disable sys_membarrier when nohz_full is enabled" > > >> > > >> "Userspace applications should be allowed to expect the membarrier system > > >> call with MEMBARRIER_CMD_SHARED command to issue memory barriers on > > >> nohz_full CPUs, but synchronize_sched() does not take those into > > >> account." > > >> > > >> So AFAIU you'd want to re-use membarrier to issue synchronize_sched, and you > > >> only care about kernel preempt off critical sections. > > >> > > >> Clearly bpf code does not run in user-space, so it would "work". > > >> > > >> But the guarantees provided by membarrier are not to synchronize against > > >> preempt off per se. It's just that the current implementation happens to > > >> do that. The point of membarrier is to turn user-space memory barriers > > >> into compiler barriers. > > >> > > >> If what you need is to wait for a RCU grace period for whatever RCU flavor > > >> ebpf is using, I would against using membarrier for this. I would rather > > >> recommend adding a dedicated BPF_SYNCHRONIZE so you won't leak > > >> implementation details to user-space, *and* you can eventually change you > > >> RCU implementation for e.g. SRCU in the future if needed. > > > > > > The point about future changes to underlying bpf mechanisms is valid. > > > There is work already on the way to reduce the scope of preempt_off+rcu_lock > > > that currently lasts the whole prog. We will have new prog types that won't > > > have such wrappers and will do rcu_lock/unlock and preempt on/off only > > > when necessary. > > > So something like BPF_SYNCHRONIZE will break soon, since the kernel cannot have > > > guarantees on when programs finish. Calling this command BPF_SYNCHRONIZE_PROG > > > also won't make sense for the same reason. > > > What we can do it instead is to define synchronization barrier for > > > programs accessing maps. May be call it something like: > > > BPF_SYNC_MAP_ACCESS ? > > > uapi/bpf.h would need to have extensive comment what this barrier is doing. > > > Implementation should probably call synchronize_rcu() and not play games > > > with synchronize_sched(), since that's going too much into implementation. > > > Also should such sys_bpf command be root only? > > > I'm not sure whether dos attack can be made by spamming synchronize_rcu() > > > and synchronize_sched() for that matter. > > > > Adding Paul E. McKenney in CC. He may want to share his thoughts on the matter. > > Let's see... > > Spamming synchronize_rcu() and synchronize_sched() should be a non-event, > at least aside from the CPUs doing the spamming. The reason for this > is that a given task can only fire off a single synchronize_sched or > synchronize_rcu() per few milliseconds, so you need a -lot- of tasks > to have much effect, at which point the sheer number of tasks is much > more a problem than the large number of outstanding synchronize_rcu() > or synchronize_sched() invocations. > > I very strongly agree that usermode should have a operation that > synchronizes with whatever eBPF uses, rather than something that forces > a specific type of RCU grace period. > > Finally, in a few releases, synchronize_sched() will be retiring in favor > of synchronize_rcu(), which will wait on preemption-disabled regions of > code in addition to waiting on RCU read-side critical sections. Not a > big deal, as I expect to enlist Coccinelle's aid in this. > > Did I manage to hit all the high points?
Thanks. It's ok for new cmd being unpriv then.
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