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btrfs: open code fc_mount() to avoid releasing s_umount rw_sempahore

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[CURRENT BEHAVIOR]
Currently inside btrfs_get_tree_subvol(), we call fc_mount() to grab a
tree, then re-lock s_umount inside btrfs_reconfigure_for_mount() to
avoid race with remount.

However fc_mount() itself is just doing two things:

1. Call vfs_get_tree()
2. Release s_umount then call vfs_create_mount()

[ENHANCEMENT]
Instead of calling fc_mount(), we can open-code it with vfs_get_tree()
first.
This provides a benefit that, since we have the full control of
s_umount, we do not need to re-lock that rw_sempahore when calling
btrfs_reconfigure_for_mount(), meaning less race between RO/RW remount.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Qu Wenruo <wqu@suse.com>
[ Rework the subject and commit message, refactor the error handling ]
Signed-off-by: Qu Wenruo <wqu@suse.com>
Tested-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Al Viro
2025-05-06 20:58:26 +01:00
committed by David Sterba
parent 4013cde56e
commit 75764b41bf
1 changed files with 17 additions and 17 deletions
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34
fs/btrfs/super.c
View File

@@ -1996,17 +1996,13 @@ static int btrfs_get_tree_super(struct fs_context *fc)
* btrfs or not, setting the whole super block RO. To make per-subvolume mounting
* work with different options work we need to keep backward compatibility.
*/
static int btrfs_reconfigure_for_mount(struct fs_context *fc, struct vfsmount *mnt)
static int btrfs_reconfigure_for_mount(struct fs_context *fc)
{
int ret = 0;
if (fc->sb_flags & SB_RDONLY)
return ret;
down_write(&mnt->mnt_sb->s_umount);
if (!(fc->sb_flags & SB_RDONLY) && (mnt->mnt_sb->s_flags & SB_RDONLY))
if (!(fc->sb_flags & SB_RDONLY) && (fc->root->d_sb->s_flags & SB_RDONLY))
ret = btrfs_reconfigure(fc);
up_write(&mnt->mnt_sb->s_umount);
return ret;
}
@@ -2059,17 +2055,18 @@ static int btrfs_get_tree_subvol(struct fs_context *fc)
security_free_mnt_opts(&fc->security);
fc->security = NULL;
mnt = fc_mount(dup_fc);
if (IS_ERR(mnt)) {
put_fs_context(dup_fc);
return PTR_ERR(mnt);
}
ret = btrfs_reconfigure_for_mount(dup_fc, mnt);
ret = vfs_get_tree(dup_fc);
if (ret)
goto error;
ret = btrfs_reconfigure_for_mount(dup_fc);
up_write(&dup_fc->root->d_sb->s_umount);
if (ret)
goto error;
mnt = vfs_create_mount(dup_fc);
put_fs_context(dup_fc);
if (ret) {
mntput(mnt);
return ret;
}
if (IS_ERR(mnt))
return PTR_ERR(mnt);
/*
* This free's ->subvol_name, because if it isn't set we have to
@@ -2083,6 +2080,9 @@ static int btrfs_get_tree_subvol(struct fs_context *fc)
fc->root = dentry;
return 0;
error:
put_fs_context(dup_fc);
return ret;
}
static int btrfs_get_tree(struct fs_context *fc)