Merge branch 'hashmap_iter_bucket_lock_fix'

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Yonghong Song says:

====================
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.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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This commit is contained in:

Alexei Starovoitov 2020-09-03 17:36:41 -07:00

parent 21e9ba5373 4daab71327

commit e6135df45e

2 changed files with 19 additions and 11 deletions

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15

kernel/bpf/hashtab.c

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15

tools/testing/selftests/bpf/progs/bpf_iter_bpf_hash_map.c

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