netdev - [RFC PATCH bpf-next 2/2] bpf, cpumap: Clean up bpf_cpu_map_entry directly in cpu_map

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Date: Fri, 28 Jul 2023 10:30:30 +0800
From: Hou Tao <houtao@...weicloud.com>
To: bpf@...r.kernel.org
Cc: netdev@...r.kernel.org,
	"David S . Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>,
	Björn Töpel <bjorn.topel@...il.com>,
	Toke Høiland-Jørgensen <toke@...hat.com>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Andrii Nakryiko <andrii@...nel.org>,
	Song Liu <song@...nel.org>,
	Hao Luo <haoluo@...gle.com>,
	Yonghong Song <yonghong.song@...ux.dev>,
	Daniel Borkmann <daniel@...earbox.net>,
	KP Singh <kpsingh@...nel.org>,
	Stanislav Fomichev <sdf@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>,
	houtao1@...wei.com
Subject: [RFC PATCH bpf-next 2/2] bpf, cpumap: Clean up bpf_cpu_map_entry directly in cpu_map_free

From: Hou Tao <houtao1@...wei.com>

After synchronize_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 empty queue and do cleanup after a RCU
grace period, do these things directly.

Signed-off-by: Hou Tao <houtao1@...wei.com>
---
 kernel/bpf/cpumap.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
index 24f39c37526f..f8e2b24320c0 100644
--- a/kernel/bpf/cpumap.c
+++ b/kernel/bpf/cpumap.c
@@ -554,16 +554,15 @@ 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 complete.
 	 */
-
 	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;
@@ -572,8 +571,8 @@ 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 */
+		/* Empty queue and do cleanup directly */
+		__cpu_map_entry_free(&rcpu->free_work.work);
 	}
 	bpf_map_area_free(cmap->cpu_map);
 	bpf_map_area_free(cmap);
-- 
2.29.2