Commit 1b7e0cae authored by Amery Hung's avatar Amery Hung Committed by Martin KaFai Lau
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bpf: Convert bpf_selem_unlink_map to failable 



To prepare for changing bpf_local_storage_map_bucket::lock to rqspinlock,
convert bpf_selem_unlink_map() to failable. It still always succeeds and
returns 0 for now.

Since some operations updating local storage cannot fail in the middle,
open-code bpf_selem_unlink_map() to take the b->lock before the
operation. There are two such locations:

- bpf_local_storage_alloc()

  The first selem will be unlinked from smap if cmpxchg owner_storage_ptr
  fails, which should not fail. Therefore, hold b->lock when linking
  until allocation complete. Helpers that assume b->lock is held by
  callers are introduced: bpf_selem_link_map_nolock() and
  bpf_selem_unlink_map_nolock().

- bpf_local_storage_update()

  The three step update process: link_map(new_selem),
  link_storage(new_selem), and unlink_map(old_selem) should not fail in
  the middle.

In bpf_selem_unlink(), bpf_selem_unlink_map() and
bpf_selem_unlink_storage() should either all succeed or fail as a whole
instead of failing in the middle. So, return if unlink_map() failed.
Remove the selem_linked_to_map_lockless() check as an selem in the
common paths (not bpf_local_storage_map_free() or
bpf_local_storage_destroy()), will be unlinked under b->lock and
local_storage->lock and therefore no other threads can unlink the selem
from map at the same time.

In bpf_local_storage_destroy(), ignore the return of
bpf_selem_unlink_map() for now. A later patch will allow
bpf_local_storage_destroy() to unlink selems even when failing to
acquire locks.

Note that while this patch removes all callers of selem_linked_to_map(),
a later patch that introduces bpf_selem_unlink_nofail() will use it
again.

Acked-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarAmery Hung <ameryhung@gmail.com>
Signed-off-by: default avatarMartin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260205222916.1788211-3-ameryhung@gmail.com
parent 0ccef707

kernel/bpf/bpf_local_storage.c

+39 −18
Original line number Diff line number Diff line
@@ -61,11 +61,6 @@ static bool selem_linked_to_storage(const struct bpf_local_storage_elem *selem) 
	return !hlist_unhashed(&selem->snode);

}



static bool selem_linked_to_map_lockless(const struct bpf_local_storage_elem *selem)

{

	return !hlist_unhashed_lockless(&selem->map_node);

}



static bool selem_linked_to_map(const struct bpf_local_storage_elem *selem)

{

	return !hlist_unhashed(&selem->map_node);
@@ -347,25 +342,27 @@ void bpf_selem_link_storage_nolock(struct bpf_local_storage *local_storage, 
	hlist_add_head_rcu(&selem->snode, &local_storage->list);

}



static void bpf_selem_unlink_map(struct bpf_local_storage_elem *selem)

static int bpf_selem_unlink_map(struct bpf_local_storage_elem *selem)

{

	struct bpf_local_storage *local_storage;

	struct bpf_local_storage_map *smap;

	struct bpf_local_storage_map_bucket *b;

	unsigned long flags;



	if (unlikely(!selem_linked_to_map_lockless(selem)))

		/* selem has already be unlinked from smap */

		return;



	local_storage = rcu_dereference_check(selem->local_storage,

					      bpf_rcu_lock_held());

	smap = rcu_dereference_check(SDATA(selem)->smap, bpf_rcu_lock_held());

	b = select_bucket(smap, local_storage);

	raw_spin_lock_irqsave(&b->lock, flags);

	if (likely(selem_linked_to_map(selem)))

	hlist_del_init_rcu(&selem->map_node);

	raw_spin_unlock_irqrestore(&b->lock, flags);



	return 0;

}



static void bpf_selem_unlink_map_nolock(struct bpf_local_storage_elem *selem)

{

	hlist_del_init_rcu(&selem->map_node);

}



void bpf_selem_link_map(struct bpf_local_storage_map *smap,
@@ -381,13 +378,24 @@ void bpf_selem_link_map(struct bpf_local_storage_map *smap, 
	raw_spin_unlock_irqrestore(&b->lock, flags);

}



static void bpf_selem_link_map_nolock(struct bpf_local_storage_map_bucket *b,

				      struct bpf_local_storage_elem *selem)

{

	hlist_add_head_rcu(&selem->map_node, &b->list);

}



void bpf_selem_unlink(struct bpf_local_storage_elem *selem, bool reuse_now)

{

	int err;



	/* Always unlink from map before unlinking from local_storage

	 * because selem will be freed after successfully unlinked from

	 * the local_storage.

	 */

	bpf_selem_unlink_map(selem);

	err = bpf_selem_unlink_map(selem);

	if (err)

		return;



	bpf_selem_unlink_storage(selem, reuse_now);

}
@@ -429,6 +437,8 @@ int bpf_local_storage_alloc(void *owner, 
{

	struct bpf_local_storage *prev_storage, *storage;

	struct bpf_local_storage **owner_storage_ptr;

	struct bpf_local_storage_map_bucket *b;

	unsigned long flags;

	int err;



	err = mem_charge(smap, owner, sizeof(*storage));
@@ -453,7 +463,10 @@ int bpf_local_storage_alloc(void *owner, 
	storage->use_kmalloc_nolock = smap->use_kmalloc_nolock;



	bpf_selem_link_storage_nolock(storage, first_selem);

	bpf_selem_link_map(smap, storage, first_selem);



	b = select_bucket(smap, storage);

	raw_spin_lock_irqsave(&b->lock, flags);

	bpf_selem_link_map_nolock(b, first_selem);



	owner_storage_ptr =

		(struct bpf_local_storage **)owner_storage(smap, owner);
@@ -469,10 +482,12 @@ int bpf_local_storage_alloc(void *owner, 
	 */

	prev_storage = cmpxchg(owner_storage_ptr, NULL, storage);

	if (unlikely(prev_storage)) {

		bpf_selem_unlink_map(first_selem);

		bpf_selem_unlink_map_nolock(first_selem);

		raw_spin_unlock_irqrestore(&b->lock, flags);

		err = -EAGAIN;

		goto uncharge;

	}

	raw_spin_unlock_irqrestore(&b->lock, flags);



	return 0;
@@ -494,8 +509,9 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap, 
	struct bpf_local_storage_data *old_sdata = NULL;

	struct bpf_local_storage_elem *alloc_selem, *selem = NULL;

	struct bpf_local_storage *local_storage;

	struct bpf_local_storage_map_bucket *b;

	HLIST_HEAD(old_selem_free_list);

	unsigned long flags;

	unsigned long flags, b_flags;

	int err;



	/* BPF_EXIST and BPF_NOEXIST cannot be both set */
@@ -579,20 +595,25 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap, 
		goto unlock;

	}



	b = select_bucket(smap, local_storage);



	raw_spin_lock_irqsave(&b->lock, b_flags);



	alloc_selem = NULL;

	/* First, link the new selem to the map */

	bpf_selem_link_map(smap, local_storage, selem);

	bpf_selem_link_map_nolock(b, selem);



	/* Second, link (and publish) the new selem to local_storage */

	bpf_selem_link_storage_nolock(local_storage, selem);



	/* Third, remove old selem, SELEM(old_sdata) */

	if (old_sdata) {

		bpf_selem_unlink_map(SELEM(old_sdata));

		bpf_selem_unlink_map_nolock(SELEM(old_sdata));

		bpf_selem_unlink_storage_nolock(local_storage, SELEM(old_sdata),

						&old_selem_free_list);

	}



	raw_spin_unlock_irqrestore(&b->lock, b_flags);

unlock:

	raw_spin_unlock_irqrestore(&local_storage->lock, flags);

	bpf_selem_free_list(&old_selem_free_list, false);