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Date:   Thu, 3 Sep 2020 17:03:18 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        bpf <bpf@...r.kernel.org>, Lorenz Bauer <lmb@...udflare.com>,
        Martin KaFai Lau <kafai@...com>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf 1/2] bpf: do not use bucket_lock for hashmap iterator

On Wed, Sep 02, 2020 at 07:44:34PM -0700, Yonghong Song wrote:
> 
> 
> On 9/2/20 6:25 PM, Andrii Nakryiko wrote:
> > On Wed, Sep 2, 2020 at 4:56 PM Yonghong Song <yhs@...com> 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@...com>
> > > ---
> > >   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()
> ...				  ...

rcu_trace doesn't protect the map. It protects the program. Even for
prog2_sleepable the map is protected by rcu. The whole map including all
elements will be freed after both sleepable and non-sleepable progs stop
executing. This rcu_read_lock is needed for non-preallocated hash maps where
individual elements are rcu protected. See free_htab_elem() doing call_rcu().
When the combination of sleepable progs and non-prealloc hashmap is enabled
we would need to revisit this iterator assumption.

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