Commit 5d800f87 authored by Amery Hung's avatar Amery Hung Committed by Martin KaFai Lau
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bpf: Support lockless unlink when freeing map or local storage 



Introduce bpf_selem_unlink_nofail() to properly handle errors returned
from rqspinlock in bpf_local_storage_map_free() and
bpf_local_storage_destroy() where the operation must succeeds.

The idea of bpf_selem_unlink_nofail() is to allow an selem to be
partially linked and use atomic operation on a bit field, selem->state,
to determine when and who can free the selem if any unlink under lock
fails. An selem initially is fully linked to a map and a local storage.
Under normal circumstances, bpf_selem_unlink_nofail() will be able to
grab locks and unlink a selem from map and local storage in sequeunce,
just like bpf_selem_unlink(), and then free it after an RCU grace period.
However, if any of the lock attempts fails, it will only clear
SDATA(selem)->smap or selem->local_storage depending on the caller and
set SELEM_MAP_UNLINKED or SELEM_STORAGE_UNLINKED according to the
caller. Then, after both map_free() and destroy() see the selem and the
state becomes SELEM_UNLINKED, one of two racing caller can succeed in
cmpxchg the state from SELEM_UNLINKED to SELEM_TOFREE, ensuring no
double free or memory leak.

To make sure bpf_obj_free_fields() is done only once and when map is
still present, it is called when unlinking an selem from b->list under
b->lock.

To make sure uncharging memory is done only when the owner is still
present in map_free(), block destroy() from returning until there is no
pending map_free().

Since smap may not be valid in destroy(), bpf_selem_unlink_nofail()
skips bpf_selem_unlink_storage_nolock_misc() when called from destroy().
This is okay as bpf_local_storage_destroy() will return the remaining
amount of memory charge tracked by mem_charge to the owner to uncharge.
It is also safe to skip clearing local_storage->owner and owner_storage
as the owner is being freed and no users or bpf programs should be able
to reference the owner and using local_storage.

Finally, access of selem, SDATA(selem)->smap and selem->local_storage
are racy. Callers will protect these fields with RCU.

Acked-by: default avatarAlexei Starovoitov <ast@kernel.org>
Co-developed-by: default avatarMartin KaFai Lau <martin.lau@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/20260205222916.1788211-11-ameryhung@gmail.com
parent c8be3da1

include/linux/bpf_local_storage.h

+8 −1
Original line number Diff line number Diff line
@@ -68,6 +68,11 @@ struct bpf_local_storage_data { 
	u8 data[] __aligned(8);

};



#define SELEM_MAP_UNLINKED	(1 << 0)

#define SELEM_STORAGE_UNLINKED	(1 << 1)

#define SELEM_UNLINKED		(SELEM_MAP_UNLINKED | SELEM_STORAGE_UNLINKED)

#define SELEM_TOFREE		(1 << 2)



/* Linked to bpf_local_storage and bpf_local_storage_map */

struct bpf_local_storage_elem {

	struct hlist_node map_node;	/* Linked to bpf_local_storage_map */
@@ -80,8 +85,9 @@ struct bpf_local_storage_elem { 
						 * after raw_spin_unlock

						 */

	};

	atomic_t state;

	bool use_kmalloc_nolock;

	/* 7 bytes hole */

	/* 3 bytes hole */

	/* The data is stored in another cacheline to minimize

	 * the number of cachelines access during a cache hit.

	 */
@@ -97,6 +103,7 @@ struct bpf_local_storage { 
	struct rcu_head rcu;

	rqspinlock_t lock;	/* Protect adding/removing from the "list" */

	u64 mem_charge;		/* Copy of mem charged to owner. Protected by "lock" */

	refcount_t owner_refcnt;/* Used to pin owner when map_free is uncharging */

	bool use_kmalloc_nolock;

};

kernel/bpf/bpf_local_storage.c

+110 −6
Original line number Diff line number Diff line
@@ -85,6 +85,7 @@ bpf_selem_alloc(struct bpf_local_storage_map *smap, void *owner, 


	if (selem) {

		RCU_INIT_POINTER(SDATA(selem)->smap, smap);

		atomic_set(&selem->state, 0);

		selem->use_kmalloc_nolock = smap->use_kmalloc_nolock;



		if (value) {
@@ -194,9 +195,11 @@ static void bpf_selem_free_rcu(struct rcu_head *rcu) 
	/* The bpf_local_storage_map_free will wait for rcu_barrier */

	smap = rcu_dereference_check(SDATA(selem)->smap, 1);



	if (smap) {

		migrate_disable();

		bpf_obj_free_fields(smap->map.record, SDATA(selem)->data);

		migrate_enable();

	}

	kfree_nolock(selem);

}
@@ -221,6 +224,7 @@ void bpf_selem_free(struct bpf_local_storage_elem *selem, 
		 * is only supported in task local storage, where

		 * smap->use_kmalloc_nolock == true.

		 */

		if (smap)

			bpf_obj_free_fields(smap->map.record, SDATA(selem)->data);

		__bpf_selem_free(selem, reuse_now);

		return;
@@ -255,7 +259,7 @@ static void bpf_selem_free_list(struct hlist_head *list, bool reuse_now) 
static void bpf_selem_unlink_storage_nolock_misc(struct bpf_local_storage_elem *selem,

						 struct bpf_local_storage_map *smap,

						 struct bpf_local_storage *local_storage,

						 bool free_local_storage)

						 bool free_local_storage, bool pin_owner)

{

	void *owner = local_storage->owner;

	u32 uncharge = smap->elem_size;
@@ -264,6 +268,9 @@ static void bpf_selem_unlink_storage_nolock_misc(struct bpf_local_storage_elem * 
	    SDATA(selem))

		RCU_INIT_POINTER(local_storage->cache[smap->cache_idx], NULL);



	if (pin_owner && !refcount_inc_not_zero(&local_storage->owner_refcnt))

		return;



	uncharge += free_local_storage ? sizeof(*local_storage) : 0;

	mem_uncharge(smap, local_storage->owner, uncharge);

	local_storage->mem_charge -= uncharge;
@@ -274,6 +281,9 @@ static void bpf_selem_unlink_storage_nolock_misc(struct bpf_local_storage_elem * 
		/* After this RCU_INIT, owner may be freed and cannot be used */

		RCU_INIT_POINTER(*owner_storage(smap, owner), NULL);

	}



	if (pin_owner)

		refcount_dec(&local_storage->owner_refcnt);

}



/* local_storage->lock must be held and selem->local_storage == local_storage.
@@ -293,7 +303,7 @@ static bool bpf_selem_unlink_storage_nolock(struct bpf_local_storage *local_stor 
						    &local_storage->list);



	bpf_selem_unlink_storage_nolock_misc(selem, smap, local_storage,

					     free_local_storage);

					     free_local_storage, false);



	hlist_del_init_rcu(&selem->snode);
@@ -409,6 +419,94 @@ int bpf_selem_unlink(struct bpf_local_storage_elem *selem, bool reuse_now) 
	return err;

}



/*

 * Unlink an selem from map and local storage with lockless fallback if callers

 * are racing or rqspinlock returns error. It should only be called by

 * bpf_local_storage_destroy() or bpf_local_storage_map_free().

 */

static void bpf_selem_unlink_nofail(struct bpf_local_storage_elem *selem,

				    struct bpf_local_storage_map_bucket *b)

{

	bool in_map_free = !!b, free_storage = false;

	struct bpf_local_storage *local_storage;

	struct bpf_local_storage_map *smap;

	unsigned long flags;

	int err, unlink = 0;



	local_storage = rcu_dereference_check(selem->local_storage, bpf_rcu_lock_held());

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



	if (smap) {

		b = b ? : select_bucket(smap, local_storage);

		err = raw_res_spin_lock_irqsave(&b->lock, flags);

		if (!err) {

			/*

			 * Call bpf_obj_free_fields() under b->lock to make sure it is done

			 * exactly once for an selem. Safe to free special fields immediately

			 * as no BPF program should be referencing the selem.

			 */

			if (likely(selem_linked_to_map(selem))) {

				hlist_del_init_rcu(&selem->map_node);

				bpf_obj_free_fields(smap->map.record, SDATA(selem)->data);

				unlink++;

			}

			raw_res_spin_unlock_irqrestore(&b->lock, flags);

		}

		/*

		 * Highly unlikely scenario: resource leak

		 *

		 * When map_free(selem1), destroy(selem1) and destroy(selem2) are racing

		 * and both selem belong to the same bucket, if destroy(selem2) acquired

		 * b->lock and block for too long, neither map_free(selem1) and

		 * destroy(selem1) will be able to free the special field associated

		 * with selem1 as raw_res_spin_lock_irqsave() returns -ETIMEDOUT.

		 */

		WARN_ON_ONCE(err && in_map_free);

		if (!err || in_map_free)

			RCU_INIT_POINTER(SDATA(selem)->smap, NULL);

	}



	if (local_storage) {

		err = raw_res_spin_lock_irqsave(&local_storage->lock, flags);

		if (!err) {

			if (likely(selem_linked_to_storage(selem))) {

				free_storage = hlist_is_singular_node(&selem->snode,

								      &local_storage->list);

				 /*

				  * Okay to skip clearing owner_storage and storage->owner in

				  * destroy() since the owner is going away. No user or bpf

				  * programs should be able to reference it.

				  */

				if (smap && in_map_free)

					bpf_selem_unlink_storage_nolock_misc(

						selem, smap, local_storage,

						free_storage, true);

				hlist_del_init_rcu(&selem->snode);

				unlink++;

			}

			raw_res_spin_unlock_irqrestore(&local_storage->lock, flags);

		}

		if (!err || !in_map_free)

			RCU_INIT_POINTER(selem->local_storage, NULL);

	}



	if (unlink != 2)

		atomic_or(in_map_free ? SELEM_MAP_UNLINKED : SELEM_STORAGE_UNLINKED, &selem->state);



	/*

	 * Normally, an selem can be unlinked under local_storage->lock and b->lock, and

	 * then freed after an RCU grace period. However, if destroy() and map_free() are

	 * racing or rqspinlock returns errors in unlikely situations (unlink != 2), free

	 * the selem only after both map_free() and destroy() see the selem.

	 */

	if (unlink == 2 ||

	    atomic_cmpxchg(&selem->state, SELEM_UNLINKED, SELEM_TOFREE) == SELEM_UNLINKED)

		bpf_selem_free(selem, true);



	if (free_storage)

		bpf_local_storage_free(local_storage, true);

}



void __bpf_local_storage_insert_cache(struct bpf_local_storage *local_storage,

				      struct bpf_local_storage_map *smap,

				      struct bpf_local_storage_elem *selem)
@@ -475,6 +573,7 @@ int bpf_local_storage_alloc(void *owner, 
	storage->owner = owner;

	storage->mem_charge = sizeof(*storage);

	storage->use_kmalloc_nolock = smap->use_kmalloc_nolock;

	refcount_set(&storage->owner_refcnt, 1);



	bpf_selem_link_storage_nolock(storage, first_selem);
@@ -743,6 +842,11 @@ void bpf_local_storage_destroy(struct bpf_local_storage *local_storage) 


	if (free_storage)

		bpf_local_storage_free(local_storage, true);



	if (!refcount_dec_and_test(&local_storage->owner_refcnt)) {

		while (refcount_read(&local_storage->owner_refcnt))

			cpu_relax();

	}

}



u64 bpf_local_storage_map_mem_usage(const struct bpf_map *map)