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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 <[email protected]>
Tested-by: Toshiyuki Okajima <[email protected]>
Signed-off-by: Al Viro <[email protected]>
This commit is contained in:
Miklos Szeredi 2011-11-21 12:11:31 +01:00 • committed by Al Viro
commit 4ed5e82fe7
4 changed files with 61 additions and 5 deletions
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1
fs/internal.h
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@ -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);