On Thu, Jan 17, 2019 at 10:14 PM Alexei Starovoitov <a...@kernel.org> wrote:
>
> Many algorithms need to read and modify several variables atomically.
> Until now it was hard to impossible to implement such algorithms in BPF.
> Hence introduce support for bpf_spin_lock.
>
> The api consists of 'struct bpf_spin_lock' that should be placed
> inside hash/array/cgroup_local_storage element
> and bpf_spin_lock/unlock() helper function.
>
> Example:
> struct hash_elem {
> int cnt;
> struct bpf_spin_lock lock;
> };
> struct hash_elem * val = bpf_map_lookup_elem(&hash_map, &key);
> if (val) {
> bpf_spin_lock(&val->lock);
> val->cnt++;
> bpf_spin_unlock(&val->lock);
> }
>
> and BPF_F_LOCK flag for lookup/update bpf syscall commands that
> allows user space to read/write map elements under lock.
>
> Together these primitives allow race free access to map elements
> from bpf programs and from user space.
>
> Key restriction: root only.
> Key requirement: maps must be annotated with BTF.
>
> This concept was discussed at Linux Plumbers Conference 2018.
> Thank you everyone who participated and helped to iron out details
> of api and implementation.
>
> Patch 1: bpf_spin_lock support in the verifier, BTF, hash, array.
> Patch 2: bpf_spin_lock in cgroup local storage.
> Patches 3,4,5: tests
> Patch 6: BPF_F_LOCK flag to lookup/update
> Patches 7,8,9: tests
>
> v1->v2:
> - addressed several issues spotted by Daniel and Martin in patch 1
> - added test11 to patch 4 as suggested by Daniel
Found a build error on ia64 when SMP is on, but qspinlock
is not used.
Will be respining the set.
