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/namespace.c b/fs/namespace.c
index 145217b..98ebc78 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -273,6 +273,15 @@
#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 @@
* 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 @@
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);
