md: resolve external metadata handling deadlock in md_allow_write

md_allow_write() marks the metadata dirty while holding mddev->lock and then
waits for the write to complete.  For externally managed metadata this causes a
deadlock as userspace needs to take the lock to communicate that the metadata
update has completed.

Change md_allow_write() in the 'external' case to start the 'mark active'
operation and then return -EAGAIN.  The expected side effects while waiting for
userspace to write 'active' to 'array_state' are holding off reshape (code
currently handles -ENOMEM), cause some 'stripe_cache_size' change requests to
fail, cause some GET_BITMAP_FILE ioctl requests to fall back to GFP_NOIO, and
cause updates to 'raid_disks' to fail.  Except for 'stripe_cache_size' changes
these failures can be mitigated by coordinating with mdmon.

md_write_start() still prevents writes from occurring until the metadata
handler has had a chance to take action as it unconditionally waits for
MD_CHANGE_CLEAN to be cleared.

[neilb@suse.de: return -EAGAIN, try GFP_NOIO]
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index f05d5983efb6..491dc2d4ad5f 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -2136,7 +2136,7 @@ static int raid1_reshape(mddev_t *mddev)
         conf_t *conf = mddev_to_conf(mddev);
         int cnt, raid_disks;
         unsigned long flags;
-        int d, d2;
+        int d, d2, err;
 
         /* Cannot change chunk_size, layout, or level */
         if (mddev->chunk_size != mddev->new_chunk ||
@@ -2148,7 +2148,9 @@ static int raid1_reshape(mddev_t *mddev)
                 return -EINVAL;
         }
 
-        md_allow_write(mddev);
+        err = md_allow_write(mddev);
+        if (err)
+                return err;
 
         raid_disks = mddev->raid_disks + mddev->delta_disks;