On Thu, Aug 09, 2018 at 09:51:49AM -0500, Mauricio Vasquez wrote: > > > Agree that existing ops are not the right alias, but deferring to user > > space as inline function also doesn't really seem like a good fit, imho, > > so I'd prefer rather to have something native. (Aside from that, the > > above inline bpf_pop() would also race between CPUs.) > > I think we should have push/pop/peek syscalls as well, having a bpf_pop() > that is race prone would create problems. Users expected maps operations to > be safe, so having one that is not will confuse them.
agree the races are not acceptable. How about a mixed solution: - introduce bpf_push/pop/peak helpers that programs will use, so they don't need to pass useless key=NULL - introduce map->ops->lookup_and_delete and map->ops->lookup_or_init that prog-side helpers can use and syscall has 1-1 mapping for Native lookup_or_init() helper for programs and syscall is badly missing. Most of the bcc scripts use it and bcc has a racy workaround. Similarly lookup_and_delete() syscall is 1-1 to pop() for stack/queue and useful for regular hash maps. At the end for stack/queue map the programs will use: int bpf_push(map, value); value_or_null = bpf_pop(map); // guaranteed non-racy for multi-cpu value_or_null = bpf_peak(map); // racy if 2+ cpus doing it from syscall: bpf_map_lookup_elem(map, NULL, &value); // returns top of stack bpf_map_lookup_and_delete_elem(map, NULL, &value); // returns top and deletes top atomically bpf_map_update_elem(map, NULL, &value); // pushes new value into stack atomically Eventually hash and other maps will implement bpf_map_lookup_and_delete() for both bpf progs and syscall. The main point that prog-side api doesn't have to match 1-1 to syscall-side, since they're different enough already. Like lookup_or_init() is badly needed for programs, but unnecessary for syscall. Thoughts?
