[PATCH] md: fix potential memalloc deadlock in md

If a GFP_KERNEL allocation is attempted in md while the mddev_lock is held,
it is possible for a deadlock to eventuate.

This happens if the array was marked 'clean', and the memalloc triggers a
write-out to the md device.

For the writeout to succeed, the array must be marked 'dirty', and that
requires getting the mddev_lock.

So, before attempting a GFP_KERNEL allocation while holding the lock, make
sure the array is marked 'dirty' (unless it is currently read-only).

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 29 insertions, 0 deletions

diff --git a/drivers/md/md.c b/drivers/md/md.c
index ec3d8e8a0bd3..e8807ea5377d 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -3564,6 +3564,8 @@ static int get_bitmap_file(mddev_t * mddev, void __user * arg)
         char *ptr, *buf = NULL;
         int err = -ENOMEM;
 
+        md_allow_write(mddev);
+
         file = kmalloc(sizeof(*file), GFP_KERNEL);
         if (!file)
                 goto out;
@@ -5032,6 +5034,33 @@ void md_write_end(mddev_t *mddev)
         }
 }
 
+/* md_allow_write(mddev)
+ * Calling this ensures that the array is marked 'active' so that writes
+ * may proceed without blocking.  It is important to call this before
+ * attempting a GFP_KERNEL allocation while holding the mddev lock.
+ * Must be called with mddev_lock held.
+ */
+void md_allow_write(mddev_t *mddev)
+{
+        if (!mddev->pers)
+                return;
+        if (mddev->ro)
+                return;
+
+        spin_lock_irq(&mddev->write_lock);
+        if (mddev->in_sync) {
+                mddev->in_sync = 0;
+                set_bit(MD_CHANGE_CLEAN, &mddev->flags);
+                if (mddev->safemode_delay &&
+                    mddev->safemode == 0)
+                        mddev->safemode = 1;
+                spin_unlock_irq(&mddev->write_lock);
+                md_update_sb(mddev, 0);
+        } else
+                spin_unlock_irq(&mddev->write_lock);
+}
+EXPORT_SYMBOL_GPL(md_allow_write);
+
 static DECLARE_WAIT_QUEUE_HEAD(resync_wait);
 
 #define SYNC_MARKS      10