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, 9 insertions, 3 deletions

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 442622067cae..8f4c70a53210 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -911,14 +911,16 @@ static int resize_stripes(raid5_conf_t *conf, int newsize)
         struct stripe_head *osh, *nsh;
         LIST_HEAD(newstripes);
         struct disk_info *ndisks;
-        int err = 0;
+        int err;
         struct kmem_cache *sc;
         int i;
 
         if (newsize <= conf->pool_size)
                 return 0; /* never bother to shrink */
 
-        md_allow_write(conf->mddev);
+        err = md_allow_write(conf->mddev);
+        if (err)
+                return err;
 
         /* Step 1 */
         sc = kmem_cache_create(conf->cache_name[1-conf->active_name],
@@ -3843,6 +3845,8 @@ raid5_store_stripe_cache_size(mddev_t *mddev, const char *page, size_t len)
 {
         raid5_conf_t *conf = mddev_to_conf(mddev);
         unsigned long new;
+        int err;
+
         if (len >= PAGE_SIZE)
                 return -EINVAL;
         if (!conf)
@@ -3858,7 +3862,9 @@ raid5_store_stripe_cache_size(mddev_t *mddev, const char *page, size_t len)
                 else
                         break;
         }
-        md_allow_write(mddev);
+        err = md_allow_write(mddev);
+        if (err)
+                return err;
         while (new > conf->max_nr_stripes) {
                 if (grow_one_stripe(conf))
                         conf->max_nr_stripes++;