Re: [PATCH 1/1] md/raid10: Remove rcu_dereference when it doesn't need rcu lock to protect

Hi Song

It can improve the performance. It needs to add rcu lock when calling
rcu_dereference.
Now it has a bug. It doesn't use rcu lock to protect. In the second
loop, it doesn't need
to use rcu_dereference when getting rdev. So to resolve this bug, we can remove
rcu_dereference directly.

Best Regards
Xiao

On Sat, Aug 14, 2021 at 12:50 AM Song Liu [off-list ref] wrote:

On Sun, Aug 8, 2021 at 9:02 PM Xiao Ni [off-list ref] wrote:

quoted

In the first loop of function raid10_handle_discard. It already
determines which disk need to handle discard request and add the
rdev reference count. So the conf->mirrors will not change until
all bios come back from underlayer disks. It doesn't need to use
rcu_dereference to get rdev.

Signed-off-by: Xiao Ni <redacted>

Will we get performance benefits from this change? If not, I would
prefer to keep the code as-is.

Thanks,
Song

quoted

---
 drivers/md/raid10.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 16977e8..cef9869 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c

@@ -1743,9 +1743,8 @@ static int raid10_handle_discard(struct mddev *mddev, struct bio *bio)
        for (disk = 0; disk < geo->raid_disks; disk++) {
                sector_t dev_start, dev_end;
                struct bio *mbio, *rbio = NULL;
-               struct md_rdev *rdev = rcu_dereference(conf->mirrors[disk].rdev);
-               struct md_rdev *rrdev = rcu_dereference(
-                       conf->mirrors[disk].replacement);
+               struct md_rdev *rdev = conf->mirrors[disk].rdev;
+               struct md_rdev *rrdev = conf->mirrors[disk].replacement;

                /*
                 * Now start to calculate the start and end address for each disk.
--

2.7.5