[PATCH 1/6] fs: unify locking semantics for fs freeze / thaw
Darrick J. Wong
djwong at kernel.org
Wed May 17 22:32:59 PDT 2023
On Sun, May 07, 2023 at 06:17:12PM -0700, Luis Chamberlain wrote:
> Right now freeze_super() and thaw_super() are called with
> different locking contexts. To expand on this is messy, so
> just unify the requirement to require grabbing an active
> reference and keep the superblock locked.
>
> Suggested-by: Christoph Hellwig <hch at infradead.org>
> Signed-off-by: Luis Chamberlain <mcgrof at kernel.org>
> ---
> block/bdev.c | 5 +++-
> fs/f2fs/gc.c | 5 ++++
> fs/gfs2/super.c | 9 +++++--
> fs/gfs2/sys.c | 6 +++++
> fs/gfs2/util.c | 5 ++++
> fs/ioctl.c | 12 ++++++++--
> fs/super.c | 62 ++++++++++++++++++-------------------------------
> 7 files changed, 60 insertions(+), 44 deletions(-)
>
> diff --git a/block/bdev.c b/block/bdev.c
> index 21c63bfef323..dc54a2a1c46e 100644
> --- a/block/bdev.c
> +++ b/block/bdev.c
> @@ -251,7 +251,7 @@ int freeze_bdev(struct block_device *bdev)
> error = sb->s_op->freeze_super(sb);
> else
> error = freeze_super(sb);
> - deactivate_super(sb);
> + deactivate_locked_super(sb);
>
> if (error) {
> bdev->bd_fsfreeze_count--;
> @@ -289,6 +289,8 @@ int thaw_bdev(struct block_device *bdev)
> sb = bdev->bd_fsfreeze_sb;
> if (!sb)
> goto out;
> + if (!get_active_super(bdev))
> + goto out;
>
> if (sb->s_op->thaw_super)
> error = sb->s_op->thaw_super(sb);
> @@ -298,6 +300,7 @@ int thaw_bdev(struct block_device *bdev)
> bdev->bd_fsfreeze_count++;
> else
> bdev->bd_fsfreeze_sb = NULL;
> + deactivate_locked_super(sb);
> out:
> mutex_unlock(&bdev->bd_fsfreeze_mutex);
> return error;
> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> index 61c5f9d26018..e31d6791d3e3 100644
> --- a/fs/f2fs/gc.c
> +++ b/fs/f2fs/gc.c
> @@ -2166,7 +2166,10 @@ int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count)
> if (err)
> return err;
>
> + if (!get_active_super(sbi->sb->s_bdev))
> + return -ENOTTY;
> freeze_super(sbi->sb);
> +
> f2fs_down_write(&sbi->gc_lock);
> f2fs_down_write(&sbi->cp_global_sem);
>
> @@ -2217,6 +2220,8 @@ int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count)
> out_err:
> f2fs_up_write(&sbi->cp_global_sem);
> f2fs_up_write(&sbi->gc_lock);
> + /* We use the same active reference from freeze */
> thaw_super(sbi->sb);
> + deactivate_locked_super(sbi->sb);
> return err;
> }
> diff --git a/fs/gfs2/super.c b/fs/gfs2/super.c
> index 5eed8c237500..e57cb593e2f3 100644
> --- a/fs/gfs2/super.c
> +++ b/fs/gfs2/super.c
> @@ -676,7 +676,12 @@ void gfs2_freeze_func(struct work_struct *work)
> struct gfs2_sbd *sdp = container_of(work, struct gfs2_sbd, sd_freeze_work);
> struct super_block *sb = sdp->sd_vfs;
>
> - atomic_inc(&sb->s_active);
> + if (!get_active_super(sb->s_bdev)) {
> + fs_info(sdp, "GFS2: couldn't grap super for thaw for filesystem\n");
> + gfs2_assert_withdraw(sdp, 0);
> + return;
> + }
> +
> error = gfs2_freeze_lock(sdp, &freeze_gh, 0);
> if (error) {
> gfs2_assert_withdraw(sdp, 0);
> @@ -690,7 +695,7 @@ void gfs2_freeze_func(struct work_struct *work)
> }
> gfs2_freeze_unlock(&freeze_gh);
> }
> - deactivate_super(sb);
> + deactivate_locked_super(sb);
> clear_bit_unlock(SDF_FS_FROZEN, &sdp->sd_flags);
> wake_up_bit(&sdp->sd_flags, SDF_FS_FROZEN);
> return;
> diff --git a/fs/gfs2/sys.c b/fs/gfs2/sys.c
> index 454dc2ff8b5e..cbb71c3520c0 100644
> --- a/fs/gfs2/sys.c
> +++ b/fs/gfs2/sys.c
> @@ -164,6 +164,9 @@ static ssize_t freeze_store(struct gfs2_sbd *sdp, const char *buf, size_t len)
> if (!capable(CAP_SYS_ADMIN))
> return -EPERM;
>
> + if (!get_active_super(sb->s_bdev))
> + return -ENOTTY;
> +
> switch (n) {
> case 0:
> error = thaw_super(sdp->sd_vfs);
> @@ -172,9 +175,12 @@ static ssize_t freeze_store(struct gfs2_sbd *sdp, const char *buf, size_t len)
> error = freeze_super(sdp->sd_vfs);
> break;
> default:
> + deactivate_locked_super(sb);
> return -EINVAL;
> }
>
> + deactivate_locked_super(sb);
> +
> if (error) {
> fs_warn(sdp, "freeze %d error %d\n", n, error);
> return error;
> diff --git a/fs/gfs2/util.c b/fs/gfs2/util.c
> index 7a6aeffcdf5c..3a0cd5e9ad84 100644
> --- a/fs/gfs2/util.c
> +++ b/fs/gfs2/util.c
> @@ -345,10 +345,15 @@ int gfs2_withdraw(struct gfs2_sbd *sdp)
> set_bit(SDF_WITHDRAW_IN_PROG, &sdp->sd_flags);
>
> if (sdp->sd_args.ar_errors == GFS2_ERRORS_WITHDRAW) {
> + if (!get_active_super(sb->s_bdev)) {
> + fs_err(sdp, "could not grab super on withdraw for file system\n");
> + return -1;
> + }
> fs_err(sdp, "about to withdraw this file system\n");
> BUG_ON(sdp->sd_args.ar_debug);
>
> signal_our_withdraw(sdp);
> + deactivate_locked_super(sb);
>
> kobject_uevent(&sdp->sd_kobj, KOBJ_OFFLINE);
>
> diff --git a/fs/ioctl.c b/fs/ioctl.c
> index 5b2481cd4750..1d20af762e0d 100644
> --- a/fs/ioctl.c
> +++ b/fs/ioctl.c
> @@ -386,6 +386,7 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
> static int ioctl_fsfreeze(struct file *filp)
> {
> struct super_block *sb = file_inode(filp)->i_sb;
> + int ret;
>
> if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
> return -EPERM;
> @@ -394,10 +395,17 @@ static int ioctl_fsfreeze(struct file *filp)
> if (sb->s_op->freeze_fs == NULL && sb->s_op->freeze_super == NULL)
> return -EOPNOTSUPP;
>
> + if (!get_active_super(sb->s_bdev))
Um... doesn't this mean that we now cannot freeze network and pseudo
filesystems?
> + return -ENOTTY;
> +
> /* Freeze */
> if (sb->s_op->freeze_super)
> - return sb->s_op->freeze_super(sb);
> - return freeze_super(sb);
> + ret = sb->s_op->freeze_super(sb);
> + ret = freeze_super(sb);
> +
> + deactivate_locked_super(sb);
> +
> + return ret;
> }
>
> static int ioctl_fsthaw(struct file *filp)
> diff --git a/fs/super.c b/fs/super.c
> index 34afe411cf2b..0e9d48846684 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -39,8 +39,6 @@
> #include <uapi/linux/mount.h>
> #include "internal.h"
>
> -static int thaw_super_locked(struct super_block *sb);
> -
> static LIST_HEAD(super_blocks);
> static DEFINE_SPINLOCK(sb_lock);
>
> @@ -851,13 +849,13 @@ struct super_block *get_active_super(struct block_device *bdev)
> if (sb->s_bdev == bdev) {
> if (!grab_super(sb))
> goto restart;
> - up_write(&sb->s_umount);
> return sb;
> }
> }
> spin_unlock(&sb_lock);
> return NULL;
> }
> +EXPORT_SYMBOL_GPL(get_active_super);
>
> struct super_block *user_get_super(dev_t dev, bool excl)
> {
> @@ -1024,13 +1022,13 @@ void emergency_remount(void)
>
> static void do_thaw_all_callback(struct super_block *sb)
> {
> - down_write(&sb->s_umount);
> + if (!get_active_super(sb->s_bdev))
> + return;
> if (sb->s_root && sb->s_flags & SB_BORN) {
> emergency_thaw_bdev(sb);
> - thaw_super_locked(sb);
> - } else {
> - up_write(&sb->s_umount);
> + thaw_super(sb);
> }
> + deactivate_locked_super(sb);
> }
>
> static void do_thaw_all(struct work_struct *work)
> @@ -1636,10 +1634,13 @@ static void sb_freeze_unlock(struct super_block *sb, int level)
> }
>
> /**
> - * freeze_super - lock the filesystem and force it into a consistent state
> + * freeze_super - force a filesystem backed by a block device into a consistent state
> * @sb: the super to lock
> *
> - * Syncs the super to make sure the filesystem is consistent and calls the fs's
> + * Used by filesystems and the kernel to freeze a fileystem backed by a block
> + * device into a consistent state. Callers must use get_active_super(bdev) to
> + * lock the @sb and when done must unlock it with deactivate_locked_super().
> + * Syncs the filesystem backed by the @sb and calls the filesystem's optional
> * freeze_fs. Subsequent calls to this without first thawing the fs will return
> * -EBUSY.
> *
> @@ -1672,22 +1673,15 @@ int freeze_super(struct super_block *sb)
> {
> int ret;
>
> - atomic_inc(&sb->s_active);
> - down_write(&sb->s_umount);
> - if (sb->s_writers.frozen != SB_UNFROZEN) {
> - deactivate_locked_super(sb);
> + if (sb->s_writers.frozen != SB_UNFROZEN)
> return -EBUSY;
> - }
>
> - if (!(sb->s_flags & SB_BORN)) {
> - up_write(&sb->s_umount);
> + if (!(sb->s_flags & SB_BORN))
> return 0; /* sic - it's "nothing to do" */
> - }
>
> if (sb_rdonly(sb)) {
> /* Nothing to do really... */
> sb->s_writers.frozen = SB_FREEZE_COMPLETE;
> - up_write(&sb->s_umount);
> return 0;
> }
>
> @@ -1707,7 +1701,6 @@ int freeze_super(struct super_block *sb)
> sb->s_writers.frozen = SB_UNFROZEN;
> sb_freeze_unlock(sb, SB_FREEZE_PAGEFAULT);
> wake_up(&sb->s_writers.wait_unfrozen);
> - deactivate_locked_super(sb);
> return ret;
> }
>
> @@ -1723,7 +1716,6 @@ int freeze_super(struct super_block *sb)
> sb->s_writers.frozen = SB_UNFROZEN;
> sb_freeze_unlock(sb, SB_FREEZE_FS);
> wake_up(&sb->s_writers.wait_unfrozen);
> - deactivate_locked_super(sb);
> return ret;
> }
> }
> @@ -1733,19 +1725,25 @@ int freeze_super(struct super_block *sb)
> */
> sb->s_writers.frozen = SB_FREEZE_COMPLETE;
> lockdep_sb_freeze_release(sb);
> - up_write(&sb->s_umount);
> return 0;
Also ... before this change, a successful freeze means that we retain
the elevated s_active during the freeze. Now the callers of this
function always call deactivate_locked_super, which decrements the
active count, even if the fs is actually frozen. The active count no
longer works the way it used to. Can that lead to freeing a frozen
super?
Alternately, I guess we could take our own s_active reference here.
--D
> }
> EXPORT_SYMBOL(freeze_super);
>
> -static int thaw_super_locked(struct super_block *sb)
> +/**
> + * thaw_super -- unlock a filesystem backed by a block device
> + * @sb: the super to thaw
> + *
> + * Used by filesystems and the kernel to thaw a fileystem backed by a block
> + * device. Callers must use get_active_super(bdev) to lock the @sb and when
> + * done must unlock it with deactivate_locked_super(). Once done, this marks
> + * the filesystem as writeable.
> + */
> +int thaw_super(struct super_block *sb)
> {
> int error;
>
> - if (sb->s_writers.frozen != SB_FREEZE_COMPLETE) {
> - up_write(&sb->s_umount);
> + if (sb->s_writers.frozen != SB_FREEZE_COMPLETE)
> return -EINVAL;
> - }
>
> if (sb_rdonly(sb)) {
> sb->s_writers.frozen = SB_UNFROZEN;
> @@ -1760,7 +1758,6 @@ static int thaw_super_locked(struct super_block *sb)
> printk(KERN_ERR
> "VFS:Filesystem thaw failed\n");
> lockdep_sb_freeze_release(sb);
> - up_write(&sb->s_umount);
> return error;
> }
> }
> @@ -1769,21 +1766,8 @@ static int thaw_super_locked(struct super_block *sb)
> sb_freeze_unlock(sb, SB_FREEZE_FS);
> out:
> wake_up(&sb->s_writers.wait_unfrozen);
> - deactivate_locked_super(sb);
> return 0;
> }
> -
> -/**
> - * thaw_super -- unlock filesystem
> - * @sb: the super to thaw
> - *
> - * Unlocks the filesystem and marks it writeable again after freeze_super().
> - */
> -int thaw_super(struct super_block *sb)
> -{
> - down_write(&sb->s_umount);
> - return thaw_super_locked(sb);
> -}
> EXPORT_SYMBOL(thaw_super);
>
> /*
> --
> 2.39.2
>
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