fs: split out functions to hold writers

When a mount is marked read-only we set MNT_WRITE_HOLD on it if there
aren't currently any active writers. Split this logic out into simple
helpers that we can use in follow-up patches.

Link: https://lore.kernel.org/r/20210121131959.646623-33-christian.brauner@ubuntu.com
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

diff --git a/fs/namespace.c b/fs/namespace.c
index 367f1c7..774ae5f 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -470,10 +470,8 @@ void mnt_drop_write_file(struct file *file)
 }
 EXPORT_SYMBOL(mnt_drop_write_file);
 
-static int mnt_make_readonly(struct mount *mnt)
+static inline int mnt_hold_writers(struct mount *mnt)
 {
-       int ret = 0;
-
        mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
        /*
         * After storing MNT_WRITE_HOLD, we'll read the counters. This store
@@ -498,15 +496,29 @@ static int mnt_make_readonly(struct mount *mnt)
         * we're counting up here.
         */
        if (mnt_get_writers(mnt) > 0)
-               ret = -EBUSY;
-       else
-               mnt->mnt.mnt_flags |= MNT_READONLY;
+               return -EBUSY;
+
+       return 0;
+}
+
+static inline void mnt_unhold_writers(struct mount *mnt)
+{
        /*
         * MNT_READONLY must become visible before ~MNT_WRITE_HOLD, so writers
         * that become unheld will see MNT_READONLY.
         */
        smp_wmb();
        mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
+}
+
+static int mnt_make_readonly(struct mount *mnt)
+{
+       int ret;
+
+       ret = mnt_hold_writers(mnt);
+       if (!ret)
+               mnt->mnt.mnt_flags |= MNT_READONLY;
+       mnt_unhold_writers(mnt);
        return ret;
 }