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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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22
fs/super.c
View File

@ -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)