bpf, cpumask: Clean up bpf_cpu_map_entry directly in cpu_map_free

After synchronous_rcu(), both the dettached XDP program and
xdp_do_flush() are completed, and the only user of bpf_cpu_map_entry
will be cpu_map_kthread_run(), so instead of calling
__cpu_map_entry_replace() to stop kthread and cleanup entry after a RCU
grace period, do these things directly.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20230816045959.358059-3-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Hou Tao

kernel/bpf/cpumap.c

@@ -566,16 +566,15 @@ static void cpu_map_free(struct bpf_map *map)
 	/* At this point bpf_prog->aux->refcnt == 0 and this map->refcnt == 0,
 	 * so the bpf programs (can be more than one that used this map) were
 	 * disconnected from events. Wait for outstanding critical sections in
-	 * these programs to complete. The rcu critical section only guarantees
-	 * no further "XDP/bpf-side" reads against bpf_cpu_map->cpu_map.
-	 * It does __not__ ensure pending flush operations (if any) are
-	 * complete.
+	 * these programs to complete. synchronize_rcu() below not only
+	 * guarantees no further "XDP/bpf-side" reads against
+	 * bpf_cpu_map->cpu_map, but also ensure pending flush operations
+	 * (if any) are completed.
 	 */
-
 	synchronize_rcu();
 
-	/* For cpu_map the remote CPUs can still be using the entries
-	 * (struct bpf_cpu_map_entry).
+	/* The only possible user of bpf_cpu_map_entry is
+	 * cpu_map_kthread_run().
 	 */
 	for (i = 0; i < cmap->map.max_entries; i++) {
 		struct bpf_cpu_map_entry *rcpu;
@@ -584,8 +583,8 @@ static void cpu_map_free(struct bpf_map *map)
 		if (!rcpu)
 			continue;
 
-		/* bq flush and cleanup happens after RCU grace-period */
-		__cpu_map_entry_replace(cmap, i, NULL); /* call_rcu */
+		/* Stop kthread and cleanup entry directly */
+		__cpu_map_entry_free(&rcpu->free_work.work);
 	}
 	bpf_map_area_free(cmap->cpu_map);
 	bpf_map_area_free(cmap);