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author | Alexei Starovoitov <ast@kernel.org> | 2020-09-04 03:36:41 +0300 |
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committer | Alexei Starovoitov <ast@kernel.org> | 2020-09-04 03:40:40 +0300 |
commit | e6135df45e21f1815a5948f452593124b1544a3e (patch) | |
tree | 1b6e2a0484ce01d82c58a29824b8c251a334fc8d /tools | |
parent | 21e9ba5373fc2cec608fd68301a1dbfd14df3172 (diff) | |
parent | 4daab7132731ac5ec9384c8a070cdb9607dc38c8 (diff) | |
download | linux-e6135df45e21f1815a5948f452593124b1544a3e.tar.xz |
Merge branch 'hashmap_iter_bucket_lock_fix'
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>
Diffstat (limited to 'tools')
-rw-r--r-- | tools/testing/selftests/bpf/progs/bpf_iter_bpf_hash_map.c | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_bpf_hash_map.c b/tools/testing/selftests/bpf/progs/bpf_iter_bpf_hash_map.c index 07ddbfdbcab7..6dfce3fd68bc 100644 --- a/tools/testing/selftests/bpf/progs/bpf_iter_bpf_hash_map.c +++ b/tools/testing/selftests/bpf/progs/bpf_iter_bpf_hash_map.c @@ -47,7 +47,10 @@ int dump_bpf_hash_map(struct bpf_iter__bpf_map_elem *ctx) __u32 seq_num = ctx->meta->seq_num; struct bpf_map *map = ctx->map; struct key_t *key = ctx->key; + struct key_t tmp_key; __u64 *val = ctx->value; + __u64 tmp_val = 0; + int ret; if (in_test_mode) { /* test mode is used by selftests to @@ -61,6 +64,18 @@ int dump_bpf_hash_map(struct bpf_iter__bpf_map_elem *ctx) if (key == (void *)0 || val == (void *)0) return 0; + /* update the value and then delete the <key, value> pair. + * it should not impact the existing 'val' which is still + * accessible under rcu. + */ + __builtin_memcpy(&tmp_key, key, sizeof(struct key_t)); + ret = bpf_map_update_elem(&hashmap1, &tmp_key, &tmp_val, 0); + if (ret) + return 0; + ret = bpf_map_delete_elem(&hashmap1, &tmp_key); + if (ret) + return 0; + key_sum_a += key->a; key_sum_b += key->b; key_sum_c += key->c; |