On Fri, Feb 8, 2019 at 2:48 PM Mauricio Vasquez <mauricio.vasq...@polito.it> wrote: > > > On 2/7/19 3:49 PM, Yonghong Song wrote: > > On Thu, Feb 7, 2019 at 8:15 AM Mauricio Vasquez > > <mauricio.vasq...@polito.it> wrote: > >> Hello folks, > >> > >> We have an eBPF program that shares a 64-bits integer with userspace > >> using an array map. The eBPF program only reads the integer while the > >> user-space application only writes it. > >> > >> We know this is possible that under some conditions, for example the > >> integer is not aligned to cache line size in x86_64, the reader can see > >> a partially updated value. We wonder if eBPF have some guarantees about > > not just cache line, you mean not in its naturally aligned boundary? > > Exactly. > > > One load/store is actually broken into multiple loads/stores by the > > compiler? > > > >> atomicity of the load operation in this case, or if this is totally > >> dependent on the target architecture. > >> > >> Is there a way to implement this without using bpf_spin_lock? > > So you mean both user space and bpf program are using bpf_spin_lock? > > Please consider this example based on BCC > https://gist.github.com/mauriciovasquezbernal/73ade7609eadcf23fb254401ed945cb3. > In this case the counter member is naturally aligned, so in x86_64 the > program will never see a partial update. However this could be not true > for all architectures. AFAIK bpf_spin_lock is the only way to implement > this example in a safe way for all architectures. > > In other words, it is not possible to make assumptions about load or > store operations atomicity in eBPF, this is dependent on the > architecture where the program is run. > > Is that statement correct?
There are some memory model discussion in the netdev mailing list. I think this shows bpf memory model is really tied to underlying arch indeed. Some kinds of lock is the only way to guarantee the atomacity. bpf_spin_lock is a kernel bpf helper. I guess user space could implement the spin_lock for map element as well based on how kernel bpf_spin_lock is implemented. > > Thanks, > > Mauricio. > > > >> Thanks, > >> > >> Mauricio. > >> > >> > >> > >> > > > > > > -=-=-=-=-=-=-=-=-=-=-=- Links: You receive all messages sent to this group. View/Reply Online (#1574): https://lists.iovisor.org/g/iovisor-dev/message/1574 Mute This Topic: https://lists.iovisor.org/mt/29691062/21656 Group Owner: iovisor-dev+ow...@lists.iovisor.org Unsubscribe: https://lists.iovisor.org/g/iovisor-dev/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
