[PATCH net] gro_cells: reduce number of synchronize_net() calls

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

After cited commit, gro_cells_destroy() became damn slow
on hosts with a lot of cores.

This is because we have one additional synchronize_net() per cpu as
stated in the changelog.

gro_cells_init() is setting NAPI_STATE_NO_BUSY_POLL, and this was enough
to not have one synchronize_net() call per netif_napi_del()

We can factorize all the synchronize_net() to a single one,
right before freeing per-cpu memory.

Fixes: 5198d545dba8 ("net: remove napi_hash_del() from driver-facing API")
Signed-off-by: Eric Dumazet <eduma...@google.com>
---
 net/core/gro_cells.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/core/gro_cells.c b/net/core/gro_cells.c
index 
e095fb871d9120787bfdf62149f4d82e0e3b0a51..6eb2e5ec2c5068e1d798557e55d084b785187a9b
 100644
--- a/net/core/gro_cells.c
+++ b/net/core/gro_cells.c
@@ -99,9 +99,14 @@ void gro_cells_destroy(struct gro_cells *gcells)
                struct gro_cell *cell = per_cpu_ptr(gcells->cells, i);
 
                napi_disable(&cell->napi);
-               netif_napi_del(&cell->napi);
+               __netif_napi_del(&cell->napi);
                __skb_queue_purge(&cell->napi_skbs);
        }
+       /* This barrier is needed because netpoll could access dev->napi_list
+        * under rcu protection.
+        */
+       synchronize_net();
+
        free_percpu(gcells->cells);
        gcells->cells = NULL;
 }
-- 
2.29.2.454.gaff20da3a2-goog