On Wed, Sep 13, 2023 at 1:26 AM Mariusz Tkaczyk
[off-list ref] wrote:

On Tue, 12 Sep 2023 15:49:12 -0700
Song Liu [off-list ref] wrote:

quoted

On Tue, Sep 12, 2023 at 6:02 AM Mariusz Tkaczyk
[off-list ref] wrote:

quoted

During working on changes proposed by Kuai [1], I determined that
mddev->active is continusly decremented for array marked by MD_CLOSING.
It brought me to md_seq_next() changed by [2]. I determined the regression
here, if mddev_get() fails we updated mddev pointer and as a result we
_put failed device.

I isolated the change in md_seq_next() and tested it- issue is no longer
reproducible but I don't see the root cause in this scenario. The bug
is obvious so I proceed with fixing. I will submit MD_CLOSING patches
separatelly.

Put the device which has been _get with previous md_seq_next() call.
Add guard for inproper mddev_put usage(). It shouldn't be called if
there are less than 1 for mddev->active.

I didn't convert atomic_t to refcount_t because refcount warns when 0 is
achieved which is likely to happen for mddev->active.

[1]
https://lore.kernel.org/linux-raid/028a21df-4397-80aa-c2a5-7c754560f595@gmail.com/T/#m6a534677d9654a4236623661c10646d45419ee1b (local)
[2] https://bugzilla.kernel.org/show_bug.cgi?id=217798

Fixes: 12a6caf27324 ("md: only delete entries from all_mddevs when the disk
is freed") Cc: Yu Kuai [off-list ref]
Cc: AceLan Kao <redacted>
Signed-off-by: Mariusz Tkaczyk <redacted>
---
 drivers/md/md.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 0fe7ab6e8ab9..bb654ff62765 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c

@@ -618,6 +618,12 @@ static void mddev_delayed_delete(struct work_struct

*ws);

 void mddev_put(struct mddev *mddev)
 {
+       /* Guard for ambiguous put. */
+       if (unlikely(atomic_read(&mddev->active) < 1)) {
+               pr_warn("%s: active refcount is lower than 1\n",
mdname(mddev));
+               return;
+       }
+

Could you please explain why we need this guard? We should probably fix
the caller that causes this.

We have an issue, so I added warning to catch similar mistakes. Please
note that for refcount_t such warning is implemented.

If you don't see it useful I can remove it.

Let's add it (or use refcount_t) in a separate patch.

quoted

quoted

        if (!atomic_dec_and_lock(&mddev->active, &all_mddevs_lock))
                return;
        if (!mddev->raid_disks && list_empty(&mddev->disks) &&

@@ -8227,19 +8233,16 @@ static void *md_seq_next(struct seq_file *seq, void

*v, loff_t *pos) {
        struct list_head *tmp;
        struct mddev *next_mddev, *mddev = v;
-       struct mddev *to_put = NULL;

IIUC, all we need is the following:

diff --git i/drivers/md/md.c w/drivers/md/md.c
index 73758b754127..a104a025084d 100644
--- i/drivers/md/md.c
+++ w/drivers/md/md.c

@@ -8265,7 +8265,7 @@ static void *md_seq_next(struct seq_file *seq,

void *v, loff_t *pos)
        spin_unlock(&all_mddevs_lock);

        if (to_put)
-               mddev_put(mddev);
+               mddev_put(to_put);
        return next_mddev;

 }

Is this sufficient? If so, how about we ship this first and refactor
the code later
in a separate patch?

That is correct it should be sufficient. If you don't want it in one patch I
will drop refactor because Kuai re-implemented it already.

Anyway, changes I made are simple and tested, I don't see it risk in it.
Your proposal will make merge conflicts less likely to appear. I that matters I
can simplify the patch.

Let's keep the fix as clean as possible. Please test the one line fix and
resubmit the patch.

Thanks,
Song