I'm sorry for the late reply, I was busy with other things and I wasn't
quite sure how I'd like this to be handled :)
On Tue 04-02-20 11:35:23, Sascha Hauer wrote:
> On Wed, Jan 29, 2020 at 01:29:29AM +0000, Al Viro wrote:
> > On Tue, Jan 28, 2020 at 11:06:31AM +0100, Sascha Hauer wrote:
> > > Hi Jan,
> >
> > > @@ -810,6 +811,36 @@ static struct super_block *quotactl_block(const char __user *special, int cmd)
> > > #endif
> > > }
> > >
> > > +static struct super_block *quotactl_path(const char __user *special, int cmd,
> > > + struct path *path)
> > > +{
> > > + struct super_block *sb;
> > > + int ret;
> > > +
> > > + ret = user_path_at(AT_FDCWD, special, LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT,
> > > + path);
> > > + if (ret)
> > > + return ERR_PTR(ret);
> > > +
> > > + sb = path->mnt->mnt_sb;
> > > +restart:
> > > + if (quotactl_cmd_onoff(cmd))
> > > + down_write(&sb->s_umount);
> > > + else
> > > + down_read(&sb->s_umount);
> > > +
> > > + if (quotactl_cmd_write(cmd) && sb->s_writers.frozen != SB_UNFROZEN) {
> > > + if (quotactl_cmd_onoff(cmd))
> > > + up_write(&sb->s_umount);
> > > + else
> > > + up_read(&sb->s_umount);
> > > + wait_event(sb->s_writers.wait_unfrozen,
> > > + sb->s_writers.frozen == SB_UNFROZEN);
> > > + goto restart;
> > > + }
> > > +
> > > + return sb;
> > > +}
> >
> > This partial duplicate of __get_super_thawed() guts does *not* belong here,
> > especially not interleaved with quota-specific checks.
> >
> > > + if (q_path) {
> > > + if (quotactl_cmd_onoff(cmd))
> > > + up_write(&sb->s_umount);
> > > + else
> > > + up_read(&sb->s_umount);
> > > +
> > > + path_put(&sb_path);
> > > + } else {
> > > + if (!quotactl_cmd_onoff(cmds))
> > > + drop_super(sb);
> > > + else
> > > + drop_super_exclusive(sb);
> > > + }
> >
> > Er... Why not have the same code that you've used to lock the damn thing
> > (needs to be moved to fs/super.c) simply get a passive ref to it? Then
> > you could do the same thing, q_path or no q_path...
>
> I am getting confused here. To an earlier version of this series you
> responded:
>
> > And for path-based you don't need to mess with superblock
> > references - just keep the struct path until the end. That
> > will keep the superblock alive and active just fine.
>
> I did that and got the objection from Jan:
>
> > So I've realized that just looking up superblock with user_path_at() is not
> > enough. Quota code also expects that the superblock will be locked
> > (sb->s_umount) and filesystem will not be frozen (in case the quota
> > operation is going to modify the filesystem). This is needed to serialize
> > e.g. remount and quota operations or quota operations among themselves.
Yes, using passive reference is not necessary. On the other hand the
symmetry with how get_super() and friends work has some appeal too so if Al
wants that, well, he's the maintainer ;)
> So after drawing circles we now seem to be back at passive references.
> What I have now in my tree is this in fs/super.c, untested currently:
I was thinking how to make the API most sensible. In the end I've decided
for a variant that is attached - we pass in struct path which enforces
active reference to a superblock and thus we don't have to be afraid of the
superblock going away or similar problems. Also the operation "get me
superblock for a path" kind of makes sense...
Guys, what do you think?
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
>From 18c7913475342f10b4723c7e22409acc573e6436 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@xxxxxxx>
Date: Tue, 18 Feb 2020 10:05:21 +0100
Subject: [PATCH] fs: Provide functions for getting locked and thawed
superblock from a path
Provide functions to get locked (with s_umount) and if desired also thawed
superblock for a given struct path. We additionally also get passive
reference count of the superblock so that the superblock can be unlocked
with put_super() / put_super_exlusive() similarly to how get_super_*()
class of functions operates.
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
fs/super.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
include/linux/fs.h | 3 +++
2 files changed, 67 insertions(+)
diff --git a/fs/super.c b/fs/super.c
index cd352530eca9..2c78eac57fa5 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -836,6 +836,70 @@ struct super_block *get_super_exclusive_thawed(struct block_device *bdev)
}
EXPORT_SYMBOL(get_super_exclusive_thawed);
+static struct super_block *__hold_super(struct path *path, bool excl,
+ bool thawed)
+{
+ struct super_block *sb = path->mnt->mnt_sb;
+
+ while (1) {
+ if (excl)
+ down_write(&sb->s_umount);
+ else
+ down_read(&sb->s_umount);
+ if (!thawed || sb->s_writers.frozen == SB_UNFROZEN) {
+ spin_lock(&sb_lock);
+ sb->s_count++;
+ spin_unlock(&sb_lock);
+ return sb;
+ }
+ if (excl)
+ up_write(&sb->s_umount);
+ else
+ up_read(&sb->s_umount);
+ wait_event(sb->s_writers.wait_unfrozen,
+ sb->s_writers.frozen == SB_UNFROZEN);
+ }
+}
+
+/**
+ * hold_super - get locked superblock for a path
+ * @path: path to get superblock for
+ *
+ * This function gets superblock for @path and returns it with s_umount
+ * held in shared mode and superblock's passive refcount (sb->s_count)
+ * incremented.
+ */
+struct super_block *hold_super(struct path *path)
+{
+ return __hold_super(path, false, false);
+}
+
+/**
+ * hold_super_thawed - get locked thawed superblock for a path
+ * @path: path to get superblock for
+ *
+ * This function gets superblock for @path, makes sure it is not frozen,
+ * and returns it with s_umount held in shared mode and superblock's
+ * passive refcount (sb->s_count) incremented.
+ */
+struct super_block *hold_super_thawed(struct path *path)
+{
+ return __hold_super(path, false, true);
+}
+
+/**
+ * hold_super_thawed_exclusive - get locked thawed superblock for a path
+ * @path: path to get superblock for
+ *
+ * This function gets superblock for @path, makes sure it is not frozen,
+ * and returns it with s_umount held in exclusive mode and superblock's
+ * passive refcount (sb->s_count) incremented.
+ */
+struct super_block *hold_super_thawed_exclusive(struct path *path)
+{
+ return __hold_super(path, true, true);
+}
+
/**
* get_active_super - get an active reference to the superblock of a device
* @bdev: device to get the superblock for
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 3cd4fe6b845e..2406494d2f54 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3305,6 +3305,9 @@ extern struct file_system_type *get_fs_type(const char *name);
extern struct super_block *get_super(struct block_device *);
extern struct super_block *get_super_thawed(struct block_device *);
extern struct super_block *get_super_exclusive_thawed(struct block_device *bdev);
+struct super_block *hold_super(struct path *path);
+struct super_block *hold_super_thawed(struct path *path);
+struct super_block *hold_super_thawed_exclusive(struct path *path);
extern struct super_block *get_active_super(struct block_device *bdev);
extern void drop_super(struct super_block *sb);
extern void drop_super_exclusive(struct super_block *sb);
--
2.16.4
