vfs: protect remounting superblock read-only

Currently remouting superblock read-only is racy in a major way.

With the per mount read-only infrastructure it is now possible to
prevent most races, which this patch attempts.

Before starting the remount read-only, iterate through all mounts
belonging to the superblock and if none of them have any pending
writes, set sb->s_readonly_remount.  This indicates that remount is in
progress and no further write requests are allowed.  If the remount
succeeds set MS_RDONLY and reset s_readonly_remount.

If the remounting is unsuccessful just reset s_readonly_remount.
This can result in transient EROFS errors, despite the fact the
remount failed.  Unfortunately hodling off writes is difficult as
remount itself may touch the filesystem (e.g. through load_nls())
which would deadlock.

A later patch deals with delayed writes due to nlink going to zero.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Tested-by: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/internal.h b/fs/internal.h
index 2523a40..9962c59 100644 (file)
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -52,6 +52,7 @@ extern int finish_automount(struct vfsmount *, struct path *);
 
 extern void mnt_make_longterm(struct vfsmount *);
 extern void mnt_make_shortterm(struct vfsmount *);
+extern int sb_prepare_remount_readonly(struct super_block *);
 
 extern void __init mnt_init(void);
 

diff --git a/fs/namespace.c b/fs/namespace.c
index 145217b..98ebc78 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -273,6 +273,15 @@ static unsigned int mnt_get_writers(struct mount *mnt)
 #endif
 }
 
+static int mnt_is_readonly(struct vfsmount *mnt)
+{
+       if (mnt->mnt_sb->s_readonly_remount)
+               return 1;
+       /* Order wrt setting s_flags/s_readonly_remount in do_remount() */
+       smp_rmb();
+       return __mnt_is_readonly(mnt);
+}
+
 /*
  * Most r/o checks on a fs are for operations that take
  * discrete amounts of time, like a write() or unlink().
@@ -312,7 +321,7 @@ int mnt_want_write(struct vfsmount *m)
         * MNT_WRITE_HOLD is cleared.
         */
        smp_rmb();
-       if (__mnt_is_readonly(m)) {
+       if (mnt_is_readonly(m)) {
                mnt_dec_writers(mnt);
                ret = -EROFS;
                goto out;
@@ -435,6 +444,35 @@ static void __mnt_unmake_readonly(struct mount *mnt)
        br_write_unlock(vfsmount_lock);
 }
 
+int sb_prepare_remount_readonly(struct super_block *sb)
+{
+       struct mount *mnt;
+       int err = 0;
+
+       br_write_lock(vfsmount_lock);
+       list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
+               if (!(mnt->mnt.mnt_flags & MNT_READONLY)) {
+                       mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
+                       smp_mb();
+                       if (mnt_get_writers(mnt) > 0) {
+                               err = -EBUSY;
+                               break;
+                       }
+               }
+       }
+       if (!err) {
+               sb->s_readonly_remount = 1;
+               smp_wmb();
+       }
+       list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
+               if (mnt->mnt.mnt_flags & MNT_WRITE_HOLD)
+                       mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
+       }
+       br_write_unlock(vfsmount_lock);
+
+       return err;
+}
+
 static void free_vfsmnt(struct mount *mnt)
 {
        kfree(mnt->mnt_devname);

diff --git a/fs/super.c b/fs/super.c
index 993ca8f..6acc022 100644 (file)
--- a/fs/super.c
+++ b/fs/super.c
@@ -723,23 +723,33 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
        /* If we are remounting RDONLY and current sb is read/write,
           make sure there are no rw files opened */
        if (remount_ro) {
-               if (force)
+               if (force) {
                        mark_files_ro(sb);
-               else if (!fs_may_remount_ro(sb))
-                       return -EBUSY;
+               } else {
+                       retval = sb_prepare_remount_readonly(sb);
+                       if (retval)
+                               return retval;
+
+                       retval = -EBUSY;
+                       if (!fs_may_remount_ro(sb))
+                               goto cancel_readonly;
+               }
        }
 
        if (sb->s_op->remount_fs) {
                retval = sb->s_op->remount_fs(sb, &flags, data);
                if (retval) {
                        if (!force)
-                               return retval;
+                               goto cancel_readonly;
                        /* If forced remount, go ahead despite any errors */
                        WARN(1, "forced remount of a %s fs returned %i\n",
                             sb->s_type->name, retval);
                }
        }
        sb->s_flags = (sb->s_flags & ~MS_RMT_MASK) | (flags & MS_RMT_MASK);
+       /* Needs to be ordered wrt mnt_is_readonly() */
+       smp_wmb();
+       sb->s_readonly_remount = 0;
 
        /*
         * Some filesystems modify their metadata via some other path than the
@@ -752,6 +762,10 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
        if (remount_ro && sb->s_bdev)
                invalidate_bdev(sb->s_bdev);
        return 0;
+
+cancel_readonly:
+       sb->s_readonly_remount = 0;
+       return retval;
 }
 
 static void do_emergency_remount(struct work_struct *work)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 03385ac..7b8a681 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1482,6 +1482,9 @@ struct super_block {
        int cleancache_poolid;
 
        struct shrinker s_shrink;       /* per-sb shrinker handle */
+
+       /* Being remounted read-only */
+       int s_readonly_remount;
 };
 
 /* superblock cache pruning functions */