[PATCH bpf 0/2] bpf: do not use bucket_lock for hashmap iterator

[Yonghong Song <yhs@fb.com>]

Currently, the bpf hashmap iterator takes a bucket_lock, a spin_lock,
before visiting each element in the bucket. This will cause a deadlock
if a map update/delete operates on an element with the same
bucket id of the visited map.

To avoid the deadlock, let us just use rcu_read_lock instead of
bucket_lock. This may result in visiting stale elements, missing some elements,
or repeating some elements, if concurrent map delete/update happens for the
same map. I think using rcu_read_lock is a reasonable compromise.
For users caring stale/missing/repeating element issues, bpf map batch
access syscall interface can be used.

Note that another approach is during bpf_iter link stage, we check
whether the iter program might be able to do update/delete to the visited
map. If it is, reject the link_create. Verifier needs to record whether
an update/delete operation happens for each map for this approach.
I just feel this checking is too specialized, hence still prefer
rcu_read_lock approach.

Patch #1 has the kernel implementation and Patch #2 added a selftest
which can trigger deadlock without Patch #1.

Yonghong Song (2):
  bpf: do not use bucket_lock for hashmap iterator
  selftests/bpf: add bpf_{update,delete}_map_elem in hashmap iter
    program

 kernel/bpf/hashtab.c                              | 15 ++++-----------
 .../selftests/bpf/progs/bpf_iter_bpf_hash_map.c   | 15 +++++++++++++++
 2 files changed, 19 insertions(+), 11 deletions(-)

-- 
2.24.1