On 9/2/20 6:25 PM, Andrii Nakryiko wrote:
On Wed, Sep 2, 2020 at 4:56 PM Yonghong Song <yhs@xxxxxx> wrote:
Currently, for hashmap, the bpf iterator will grab a bucket lock, a
spinlock, before traversing the elements in the bucket. This can ensure
all bpf visted elements are valid. But this mechanism may cause
deadlock if update/deletion happens to the same bucket of the
visited map in the program. For example, if we added bpf_map_update_elem()
call to the same visited element in selftests bpf_iter_bpf_hash_map.c,
we will have the following deadlock:
[...]
Compared to old bucket_lock mechanism, if concurrent updata/delete happens,
we may visit stale elements, miss some elements, or repeat some elements.
I think this is a reasonable compromise. For users wanting to avoid
I agree, the only reliable way to iterate map without duplicates and
missed elements is to not update that map during iteration (unless we
start supporting point-in-time snapshots, which is a very different
matter).
stale, missing/repeated accesses, bpf_map batch access syscall interface
can be used.
Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
kernel/bpf/hashtab.c | 15 ++++-----------
1 file changed, 4 insertions(+), 11 deletions(-)
diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 78dfff6a501b..7df28a45c66b 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -1622,7 +1622,6 @@ struct bpf_iter_seq_hash_map_info {
struct bpf_map *map;
struct bpf_htab *htab;
void *percpu_value_buf; // non-zero means percpu hash
- unsigned long flags;
u32 bucket_id;
u32 skip_elems;
};
@@ -1632,7 +1631,6 @@ bpf_hash_map_seq_find_next(struct bpf_iter_seq_hash_map_info *info,
struct htab_elem *prev_elem)
{
const struct bpf_htab *htab = info->htab;
- unsigned long flags = info->flags;
u32 skip_elems = info->skip_elems;
u32 bucket_id = info->bucket_id;
struct hlist_nulls_head *head;
@@ -1656,19 +1654,18 @@ bpf_hash_map_seq_find_next(struct bpf_iter_seq_hash_map_info *info,
/* not found, unlock and go to the next bucket */
b = &htab->buckets[bucket_id++];
- htab_unlock_bucket(htab, b, flags);
+ rcu_read_unlock();
Just double checking as I don't yet completely understand all the
sleepable BPF implications. If the map is used from a sleepable BPF
program, we are still ok doing just rcu_read_lock/rcu_read_unlock when
accessing BPF map elements, right? No need for extra
rcu_read_lock_trace/rcu_read_unlock_trace?
I think it is fine now since currently bpf_iter program cannot be
sleepable and the current sleepable program framework already allows the
following scenario.
- map1 is a preallocated hashmap shared by two programs,
prog1_nosleep and prog2_sleepable
... ...
rcu_read_lock() rcu_read_lock_trace()
run prog1_nosleep run prog2_sleepable
lookup/update/delete map1 elem lookup/update/delete map1 elem
rcu_read_unlock() rcu_read_unlock_trace()
... ...
The prog1_nosleep could be a bpf_iter program or a networking problem.
Alexei, could you confirm the above scenario is properly supported now?
skip_elems = 0;
}
[...]
