commit fda64ae0bb3e37b5a4292625c6931cb156224d0f
author Martin KaFai Lau <martin.lau@kernel.org> Tue Oct 25 11:45:21 2022 -0700
committer Alexei Starovoitov <ast@kernel.org> Tue Oct 25 23:11:46 2022 -0700
tree 27fa66933685435b9a643e29b0fb0c9aa3b4c14a
parent 4279adb094a17132423f1271c3d11b593fc2327e

bpf: bpf_task_storage_delete_recur does lookup first before the deadlock check

Similar to the earlier change in bpf_task_storage_get_recur.
This patch changes bpf_task_storage_delete_recur such that it
does the lookup first.  It only returns -EBUSY if it needs to
take the spinlock to do the deletion when potential deadlock
is detected.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20221025184524.3526117-7-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/bpf_task_storage.c

diff --git a/kernel/bpf/bpf_task_storage.c b/kernel/bpf/bpf_task_storage.c
index c3a841b..f3f79b6 100644
--- a/kernel/bpf/bpf_task_storage.c
+++ b/kernel/bpf/bpf_task_storage.c
@@ -184,7 +184,8 @@ static int bpf_pid_task_storage_update_elem(struct bpf_map *map, void *key,
 	return err;
 }
 
-static int task_storage_delete(struct task_struct *task, struct bpf_map *map)
+static int task_storage_delete(struct task_struct *task, struct bpf_map *map,
+			       bool nobusy)
 {
 	struct bpf_local_storage_data *sdata;
 
@@ -192,6 +193,9 @@ static int task_storage_delete(struct task_struct *task, struct bpf_map *map)
 	if (!sdata)
 		return -ENOENT;
 
+	if (!nobusy)
+		return -EBUSY;
+
 	bpf_selem_unlink(SELEM(sdata), true);
 
 	return 0;
@@ -220,7 +224,7 @@ static int bpf_pid_task_storage_delete_elem(struct bpf_map *map, void *key)
 	}
 
 	bpf_task_storage_lock();
-	err = task_storage_delete(task, map);
+	err = task_storage_delete(task, map, true);
 	bpf_task_storage_unlock();
 out:
 	put_pid(pid);
@@ -289,21 +293,21 @@ BPF_CALL_5(bpf_task_storage_get, struct bpf_map *, map, struct task_struct *,
 BPF_CALL_2(bpf_task_storage_delete_recur, struct bpf_map *, map, struct task_struct *,
 	   task)
 {
+	bool nobusy;
 	int ret;
 
 	WARN_ON_ONCE(!bpf_rcu_lock_held());
 	if (!task)
 		return -EINVAL;
 
-	if (!bpf_task_storage_trylock())
-		return -EBUSY;
-
+	nobusy = bpf_task_storage_trylock();
 	/* This helper must only be called from places where the lifetime of the task
 	 * is guaranteed. Either by being refcounted or by being protected
 	 * by an RCU read-side critical section.
 	 */
-	ret = task_storage_delete(task, map);
-	bpf_task_storage_unlock();
+	ret = task_storage_delete(task, map, nobusy);
+	if (nobusy)
+		bpf_task_storage_unlock();
 	return ret;
 }