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fs: Provide helpers for manipulating sb->s_readonly_remount

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Provide helpers to set and clear sb->s_readonly_remount including
appropriate memory barriers. Also use this opportunity to document what
the barriers pair with and why they are needed.

Suggested-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Message-Id: <20230620112832.5158-1-jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Jan Kara 2023-06-20 13:28:32 +02:00 committed by Christian Brauner
parent c541dce86c
commit d7439fb1f4
4 changed files with 64 additions and 21 deletions
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41
fs/internal.h
View file

@ -120,6 +120,47 @@ void put_super(struct super_block *sb);
extern bool mount_capable(struct fs_context *);
int sb_init_dio_done_wq(struct super_block *sb);
/*
* Prepare superblock for changing its read-only state (i.e., either remount
* read-write superblock read-only or vice versa). After this function returns
* mnt_is_readonly() will return true for any mount of the superblock if its
* caller is able to observe any changes done by the remount. This holds until
* sb_end_ro_state_change() is called.
*/
static inline void sb_start_ro_state_change(struct super_block *sb)
{
WRITE_ONCE(sb->s_readonly_remount, 1);
/*
* For RO->RW transition, the barrier pairs with the barrier in
* mnt_is_readonly() making sure if mnt_is_readonly() sees SB_RDONLY
* cleared, it will see s_readonly_remount set.
* For RW->RO transition, the barrier pairs with the barrier in
* __mnt_want_write() before the mnt_is_readonly() check. The barrier
* makes sure if __mnt_want_write() sees MNT_WRITE_HOLD already
* cleared, it will see s_readonly_remount set.
*/
smp_wmb();
}
/*
* Ends section changing read-only state of the superblock. After this function
* returns if mnt_is_readonly() returns false, the caller will be able to
* observe all the changes remount did to the superblock.
*/
static inline void sb_end_ro_state_change(struct super_block *sb)
{
/*
* This barrier provides release semantics that pairs with
* the smp_rmb() acquire semantics in mnt_is_readonly().
* This barrier pair ensure that when mnt_is_readonly() sees
* 0 for sb->s_readonly_remount, it will also see all the
* preceding flag changes that were made during the RO state
* change.
*/
smp_wmb();
WRITE_ONCE(sb->s_readonly_remount, 0);
}
/*
* open.c
*/

25
fs/namespace.c
View file

@ -309,9 +309,16 @@ static unsigned int mnt_get_writers(struct mount *mnt)
static int mnt_is_readonly(struct vfsmount *mnt)
{
if (mnt->mnt_sb->s_readonly_remount)
if (READ_ONCE(mnt->mnt_sb->s_readonly_remount))
return 1;
/* Order wrt setting s_flags/s_readonly_remount in do_remount() */
/*
* The barrier pairs with the barrier in sb_start_ro_state_change()
* making sure if we don't see s_readonly_remount set yet, we also will
* not see any superblock / mount flag changes done by remount.
* It also pairs with the barrier in sb_end_ro_state_change()
* assuring that if we see s_readonly_remount already cleared, we will
* see the values of superblock / mount flags updated by remount.
*/
smp_rmb();
return __mnt_is_readonly(mnt);
}
@ -364,9 +371,11 @@ int __mnt_want_write(struct vfsmount *m)
}
}
/*
* After the slowpath clears MNT_WRITE_HOLD, mnt_is_readonly will
* be set to match its requirements. So we must not load that until
* MNT_WRITE_HOLD is cleared.
* The barrier pairs with the barrier sb_start_ro_state_change() making
* sure that if we see MNT_WRITE_HOLD cleared, we will also see
* s_readonly_remount set (or even SB_RDONLY / MNT_READONLY flags) in
* mnt_is_readonly() and bail in case we are racing with remount
* read-only.
*/
smp_rmb();
if (mnt_is_readonly(m)) {
@ -588,10 +597,8 @@ int sb_prepare_remount_readonly(struct super_block *sb)
if (!err && atomic_long_read(&sb->s_remove_count))
err = -EBUSY;
if (!err) {
sb->s_readonly_remount = 1;
smp_wmb();
}
if (!err)
sb_start_ro_state_change(sb);
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;

17
fs/super.c
View file

@ -944,8 +944,7 @@ int reconfigure_super(struct fs_context *fc)
*/
if (remount_ro) {
if (force) {
sb->s_readonly_remount = 1;
smp_wmb();
sb_start_ro_state_change(sb);
} else {
retval = sb_prepare_remount_readonly(sb);
if (retval)
@ -953,12 +952,10 @@ int reconfigure_super(struct fs_context *fc)
}
} else if (remount_rw) {
/*
* We set s_readonly_remount here to protect filesystem's
* reconfigure code from writes from userspace until
* reconfigure finishes.
* Protect filesystem's reconfigure code from writes from
* userspace until reconfigure finishes.
*/
sb->s_readonly_remount = 1;
smp_wmb();
sb_start_ro_state_change(sb);
}
if (fc->ops->reconfigure) {
@ -974,9 +971,7 @@ int reconfigure_super(struct fs_context *fc)
WRITE_ONCE(sb->s_flags, ((sb->s_flags & ~fc->sb_flags_mask) |
(fc->sb_flags & fc->sb_flags_mask)));
/* Needs to be ordered wrt mnt_is_readonly() */
smp_wmb();
sb->s_readonly_remount = 0;
sb_end_ro_state_change(sb);
/*
* Some filesystems modify their metadata via some other path than the
@ -991,7 +986,7 @@ int reconfigure_super(struct fs_context *fc)
return 0;
cancel_readonly:
sb->s_readonly_remount = 0;
sb_end_ro_state_change(sb);
return retval;
}

2
include/linux/fs.h
View file

@ -1248,7 +1248,7 @@ struct super_block {
*/
atomic_long_t s_fsnotify_connectors;
/* Being remounted read-only */
/* Read-only state of the superblock is being changed */
int s_readonly_remount;
/* per-sb errseq_t for reporting writeback errors via syncfs */