[PATCH 1/2] scsi: target: alua: Don't wait for a grace period when attaching a LUN

[Qiliang Yuan <qiliang.yuan@linux.dev>]

core_tpg_add_lun() attaches a new LUN to the default ALUA target port
group before adding it to the device and TPG lists, and
target_attach_tg_pt_gp() then waits for an RCU grace period.

The wait protects nothing: lun->lun_tg_pt_gp goes from NULL to a live
group, nothing is freed, and no reader can reach the LUN yet. It still
costs a grace period for every LUN created through configfs, which
dominates the time spent exporting many LUNs.

Remove the synchronize_rcu() call. Group changes through
core_alua_store_tg_pt_gp_info() keep their own grace period.

Adding 256 LUNs to an iSCSI TPG through configfs, five rounds
alternating with and without this patch, takes (median, min-max in
parentheses):

  LUNs  before (ms)       after (ms)        change
   256  3144 (3104-3175)  1379 (1277-1393)    -56%

Signed-off-by: Qiliang Yuan <qiliang.yuan@linux.dev>
---
 drivers/target/target_core_alua.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/target/target_core_alua.c b/drivers/target/target_core_alua.c
index 140154d93c430..35d36cd36db6c 100644
--- a/drivers/target/target_core_alua.c
+++ b/drivers/target/target_core_alua.c
@@ -1833,7 +1833,6 @@ void target_attach_tg_pt_gp(struct se_lun *lun,
 	spin_lock(&lun->lun_tg_pt_gp_lock);
 	__target_attach_tg_pt_gp(lun, tg_pt_gp);
 	spin_unlock(&lun->lun_tg_pt_gp_lock);
-	synchronize_rcu();
 }
 
 static void __target_detach_tg_pt_gp(struct se_lun *lun,

-- 
2.43.0