[PATCH v5 bpf-next 05/12] bpf: Postpone bpf_obj_free_fields to the rcu callback

[Martin KaFai Lau <martin.lau@linux.dev>]

From: Martin KaFai Lau <martin.lau@kernel.org>

A later patch will enable the uptr usage in the task_local_storage map.
This will require the unpin_user_page() to be done after the rcu
task trace gp for the cases that the uptr may still be used by
a bpf prog. The bpf_obj_free_fields() will be the one doing
unpin_user_page(), so this patch is to postpone calling
bpf_obj_free_fields() to the rcu callback.

The bpf_obj_free_fields() is only required to be done in
the rcu callback when bpf->bpf_ma==true and reuse_now==false.

bpf->bpf_ma==true case is because uptr will only be enabled
in task storage which has already been moved to bpf_mem_alloc.
The bpf->bpf_ma==false case can be supported in the future
also if there is a need.

reuse_now==false when the selem (aka storage) is deleted
by bpf prog (bpf_task_storage_delete) or by syscall delete_elem().
In both cases, bpf_obj_free_fields() needs to wait for
rcu gp.

A few words on reuse_now==true. reuse_now==true when the
storage's owner (i.e. the task_struct) is destructing or the map
itself is doing map_free(). In both cases, no bpf prog should
have a hold on the selem and its uptrs, so there is no need to
postpone bpf_obj_free_fields(). reuse_now==true should be the
common case for local storage usage where the storage exists
throughout the lifetime of its owner (task_struct).

The bpf_obj_free_fields() needs to use the map->record. Doing
bpf_obj_free_fields() in a rcu callback will require the
bpf_local_storage_map_free() to wait for rcu_barrier. An optimization
could be only waiting for rcu_barrier when the map has uptr in
its map_value. This will require either yet another rcu callback
function or adding a bool in the selem to flag if the SDATA(selem)->smap
is still valid. This patch chooses to keep it simple and wait for
rcu_barrier for maps that use bpf_mem_alloc.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
---
 kernel/bpf/bpf_local_storage.c | 29 ++++++++++++++++++++++++-----
 1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
index 09a67dff2336..ca871be1c42d 100644
--- a/kernel/bpf/bpf_local_storage.c
+++ b/kernel/bpf/bpf_local_storage.c
@@ -209,8 +209,12 @@ static void __bpf_selem_free(struct bpf_local_storage_elem *selem,
 static void bpf_selem_free_rcu(struct rcu_head *rcu)
 {
 	struct bpf_local_storage_elem *selem;
+	struct bpf_local_storage_map *smap;
 
 	selem = container_of(rcu, struct bpf_local_storage_elem, rcu);
+	/* The bpf_local_storage_map_free will wait for rcu_barrier */
+	smap = rcu_dereference_check(SDATA(selem)->smap, 1);
+	bpf_obj_free_fields(smap->map.record, SDATA(selem)->data);
 	bpf_mem_cache_raw_free(selem);
 }
 
@@ -226,16 +230,25 @@ void bpf_selem_free(struct bpf_local_storage_elem *selem,
 		    struct bpf_local_storage_map *smap,
 		    bool reuse_now)
 {
-	bpf_obj_free_fields(smap->map.record, SDATA(selem)->data);
-
 	if (!smap->bpf_ma) {
+		/* Only task storage has uptrs and task storage
+		 * has moved to bpf_mem_alloc. Meaning smap->bpf_ma == true
+		 * for task storage, so this bpf_obj_free_fields() won't unpin
+		 * any uptr.
+		 */
+		bpf_obj_free_fields(smap->map.record, SDATA(selem)->data);
 		__bpf_selem_free(selem, reuse_now);
 		return;
 	}
 
-	if (!reuse_now) {
-		call_rcu_tasks_trace(&selem->rcu, bpf_selem_free_trace_rcu);
-	} else {
+	if (reuse_now) {
+		/* reuse_now == true only happens when the storage owner
+		 * (e.g. task_struct) is being destructed or the map itself
+		 * is being destructed (ie map_free). In both cases,
+		 * no bpf prog can have a hold on the selem. It is
+		 * safe to unpin the uptrs and free the selem now.
+		 */
+		bpf_obj_free_fields(smap->map.record, SDATA(selem)->data);
 		/* Instead of using the vanilla call_rcu(),
 		 * bpf_mem_cache_free will be able to reuse selem
 		 * immediately.
@@ -243,7 +256,10 @@ void bpf_selem_free(struct bpf_local_storage_elem *selem,
 		migrate_disable();
 		bpf_mem_cache_free(&smap->selem_ma, selem);
 		migrate_enable();
+		return;
 	}
+
+	call_rcu_tasks_trace(&selem->rcu, bpf_selem_free_trace_rcu);
 }
 
 static void bpf_selem_free_list(struct hlist_head *list, bool reuse_now)
@@ -908,6 +924,9 @@ void bpf_local_storage_map_free(struct bpf_map *map,
 	synchronize_rcu();
 
 	if (smap->bpf_ma) {
+		rcu_barrier_tasks_trace();
+		if (!rcu_trace_implies_rcu_gp())
+			rcu_barrier();
 		bpf_mem_alloc_destroy(&smap->selem_ma);
 		bpf_mem_alloc_destroy(&smap->storage_ma);
 	}
-- 
2.43.5