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vfs: protect remounting superblock read-only

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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>
This commit is contained in:
Miklos Szeredi
2011-11-21 12:11:31 +01:00
committed by Al Viro
parent 39f7c4db1d
commit 4ed5e82fe7
4 changed files with 61 additions and 5 deletions
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40
fs/namespace.c
View File

@@ -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);