Re: [PATCH] fs: Eliminate a local variable to make the code more clear
On Wed, Sep 09, 2020 at 07:54:44AM -0500, Eric W. Biederman wrote:
> Hao Lee writes:
>
> > On Tue, Sep 08, 2020 at 07:48:57PM +0100, Al Viro wrote:
> >> On Tue, Sep 08, 2020 at 01:06:56PM +, Hao Lee wrote:
> >> > ping
> >> >
> >> > On Wed, Jul 29, 2020 at 03:21:28PM +, Hao Lee wrote:
> >> > > The dentry local variable is introduced in 'commit 84d17192d2afd ("get
> >> > > rid of full-hash scan on detaching vfsmounts")' to reduce the length of
> >> > > some long statements for example
> >> > > mutex_lock(>dentry->d_inode->i_mutex). We have already used
> >> > > inode_lock(dentry->d_inode) to do the same thing now, and its length is
> >> > > acceptable. Furthermore, it seems not concise that assign path->dentry
> >> > > to local variable dentry in the statement before goto. So, this
> >> > > function
> >> > > would be more clear if we eliminate the local variable dentry.
> >>
> >> How does it make the function more clear? More specifically, what
> >> analysis of behaviour is simplified by that?
> >
> > When I first read this function, it takes me a few seconds to think
> > about if the local variable dentry is always equal to path->dentry and
> > want to know if it has special purpose. This local variable may confuse
> > other people too, so I think it would be better to eliminate it.
>
> I tend to have the opposite reaction. I read your patch and wonder
> why path->dentry needs to be reread what is changing path that I can not see.
> my back.
If I understand correctly, accessing path->dentry->d_inode needs one
more instruction than accessing dentry->d_inode, so the original code is
more efficient.
>
> Now for clarity it would probably help to do something like:
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index bae0e95b3713..430f3b4785e3 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2206,7 +2206,7 @@ static struct mountpoint *lock_mount(struct path *path)
> return mp;
> }
> namespace_unlock();
> - inode_unlock(path->dentry->d_inode);
> + inode_unlock(dentry->d_inode);
> path_put(path);
> path->mnt = mnt;
> dentry = path->dentry = dget(mnt->mnt_root);
>
>
> So at least the inode_lock and inode_unlock are properly paired.
>
> At first glance inode_unlock using path->dentry instead of dentry
> appears to be an oversight in 84d17192d2af ("get rid of full-hash scan
> on detaching vfsmounts").
I think I have understood why we use the local variable dentry. Thanks.
Regards,
Hao Lee
>
>
> Eric
Re: [PATCH] fs: Eliminate a local variable to make the code more clear
Hao Lee writes:
> On Tue, Sep 08, 2020 at 07:48:57PM +0100, Al Viro wrote:
>> On Tue, Sep 08, 2020 at 01:06:56PM +, Hao Lee wrote:
>> > ping
>> >
>> > On Wed, Jul 29, 2020 at 03:21:28PM +, Hao Lee wrote:
>> > > The dentry local variable is introduced in 'commit 84d17192d2afd ("get
>> > > rid of full-hash scan on detaching vfsmounts")' to reduce the length of
>> > > some long statements for example
>> > > mutex_lock(>dentry->d_inode->i_mutex). We have already used
>> > > inode_lock(dentry->d_inode) to do the same thing now, and its length is
>> > > acceptable. Furthermore, it seems not concise that assign path->dentry
>> > > to local variable dentry in the statement before goto. So, this function
>> > > would be more clear if we eliminate the local variable dentry.
>>
>> How does it make the function more clear? More specifically, what
>> analysis of behaviour is simplified by that?
>
> When I first read this function, it takes me a few seconds to think
> about if the local variable dentry is always equal to path->dentry and
> want to know if it has special purpose. This local variable may confuse
> other people too, so I think it would be better to eliminate it.
I tend to have the opposite reaction. I read your patch and wonder
why path->dentry needs to be reread what is changing path that I can not see.
my back.
Now for clarity it would probably help to do something like:
diff --git a/fs/namespace.c b/fs/namespace.c
index bae0e95b3713..430f3b4785e3 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2206,7 +2206,7 @@ static struct mountpoint *lock_mount(struct path *path)
return mp;
}
namespace_unlock();
- inode_unlock(path->dentry->d_inode);
+ inode_unlock(dentry->d_inode);
path_put(path);
path->mnt = mnt;
dentry = path->dentry = dget(mnt->mnt_root);
So at least the inode_lock and inode_unlock are properly paired.
At first glance inode_unlock using path->dentry instead of dentry
appears to be an oversight in 84d17192d2af ("get rid of full-hash scan
on detaching vfsmounts").
Eric
Re: [PATCH] fs: Eliminate a local variable to make the code more clear
On Tue, Sep 08, 2020 at 07:48:57PM +0100, Al Viro wrote:
> On Tue, Sep 08, 2020 at 01:06:56PM +, Hao Lee wrote:
> > ping
> >
> > On Wed, Jul 29, 2020 at 03:21:28PM +, Hao Lee wrote:
> > > The dentry local variable is introduced in 'commit 84d17192d2afd ("get
> > > rid of full-hash scan on detaching vfsmounts")' to reduce the length of
> > > some long statements for example
> > > mutex_lock(>dentry->d_inode->i_mutex). We have already used
> > > inode_lock(dentry->d_inode) to do the same thing now, and its length is
> > > acceptable. Furthermore, it seems not concise that assign path->dentry
> > > to local variable dentry in the statement before goto. So, this function
> > > would be more clear if we eliminate the local variable dentry.
>
> How does it make the function more clear? More specifically, what
> analysis of behaviour is simplified by that?
When I first read this function, it takes me a few seconds to think
about if the local variable dentry is always equal to path->dentry and
want to know if it has special purpose. This local variable may confuse
other people too, so I think it would be better to eliminate it.
Thanks,
Hao Lee
Re: [PATCH] fs: Eliminate a local variable to make the code more clear
ping
On Wed, Jul 29, 2020 at 03:21:28PM +, Hao Lee wrote:
> The dentry local variable is introduced in 'commit 84d17192d2afd ("get
> rid of full-hash scan on detaching vfsmounts")' to reduce the length of
> some long statements for example
> mutex_lock(>dentry->d_inode->i_mutex). We have already used
> inode_lock(dentry->d_inode) to do the same thing now, and its length is
> acceptable. Furthermore, it seems not concise that assign path->dentry
> to local variable dentry in the statement before goto. So, this function
> would be more clear if we eliminate the local variable dentry.
>
> The function logic is not changed.
>
> Signed-off-by: Hao Lee
> ---
> fs/namespace.c | 13 ++---
> 1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 4a0f600a3328..fcb93586fcc9 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2187,20 +2187,19 @@ static int attach_recursive_mnt(struct mount
> *source_mnt,
> static struct mountpoint *lock_mount(struct path *path)
> {
> struct vfsmount *mnt;
> - struct dentry *dentry = path->dentry;
> retry:
> - inode_lock(dentry->d_inode);
> - if (unlikely(cant_mount(dentry))) {
> - inode_unlock(dentry->d_inode);
> + inode_lock(path->dentry->d_inode);
> + if (unlikely(cant_mount(path->dentry))) {
> + inode_unlock(path->dentry->d_inode);
> return ERR_PTR(-ENOENT);
> }
> namespace_lock();
> mnt = lookup_mnt(path);
> if (likely(!mnt)) {
> - struct mountpoint *mp = get_mountpoint(dentry);
> + struct mountpoint *mp = get_mountpoint(path->dentry);
> if (IS_ERR(mp)) {
> namespace_unlock();
> - inode_unlock(dentry->d_inode);
> + inode_unlock(path->dentry->d_inode);
> return mp;
> }
> return mp;
> @@ -2209,7 +2208,7 @@ static struct mountpoint *lock_mount(struct path *path)
> inode_unlock(path->dentry->d_inode);
> path_put(path);
> path->mnt = mnt;
> - dentry = path->dentry = dget(mnt->mnt_root);
> + path->dentry = dget(mnt->mnt_root);
> goto retry;
> }
>
> --
> 2.24.1
>
Re: [PATCH] fs: Eliminate a local variable to make the code more clear
On Tue, Sep 08, 2020 at 01:06:56PM +, Hao Lee wrote:
> ping
>
> On Wed, Jul 29, 2020 at 03:21:28PM +, Hao Lee wrote:
> > The dentry local variable is introduced in 'commit 84d17192d2afd ("get
> > rid of full-hash scan on detaching vfsmounts")' to reduce the length of
> > some long statements for example
> > mutex_lock(>dentry->d_inode->i_mutex). We have already used
> > inode_lock(dentry->d_inode) to do the same thing now, and its length is
> > acceptable. Furthermore, it seems not concise that assign path->dentry
> > to local variable dentry in the statement before goto. So, this function
> > would be more clear if we eliminate the local variable dentry.
How does it make the function more clear? More specifically, what
analysis of behaviour is simplified by that?
Re: [PATCH] fs: Eliminate a local variable to make the code more clear
Ping
On Wed, Jul 29, 2020 at 11:21 PM Hao Lee wrote:
>
> The dentry local variable is introduced in 'commit 84d17192d2afd ("get
> rid of full-hash scan on detaching vfsmounts")' to reduce the length of
> some long statements for example
> mutex_lock(>dentry->d_inode->i_mutex). We have already used
> inode_lock(dentry->d_inode) to do the same thing now, and its length is
> acceptable. Furthermore, it seems not concise that assign path->dentry
> to local variable dentry in the statement before goto. So, this function
> would be more clear if we eliminate the local variable dentry.
>
> The function logic is not changed.
>
> Signed-off-by: Hao Lee
> ---
> fs/namespace.c | 13 ++---
> 1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 4a0f600a3328..fcb93586fcc9 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2187,20 +2187,19 @@ static int attach_recursive_mnt(struct mount
> *source_mnt,
> static struct mountpoint *lock_mount(struct path *path)
> {
> struct vfsmount *mnt;
> - struct dentry *dentry = path->dentry;
> retry:
> - inode_lock(dentry->d_inode);
> - if (unlikely(cant_mount(dentry))) {
> - inode_unlock(dentry->d_inode);
> + inode_lock(path->dentry->d_inode);
> + if (unlikely(cant_mount(path->dentry))) {
> + inode_unlock(path->dentry->d_inode);
> return ERR_PTR(-ENOENT);
> }
> namespace_lock();
> mnt = lookup_mnt(path);
> if (likely(!mnt)) {
> - struct mountpoint *mp = get_mountpoint(dentry);
> + struct mountpoint *mp = get_mountpoint(path->dentry);
> if (IS_ERR(mp)) {
> namespace_unlock();
> - inode_unlock(dentry->d_inode);
> + inode_unlock(path->dentry->d_inode);
> return mp;
> }
> return mp;
> @@ -2209,7 +2208,7 @@ static struct mountpoint *lock_mount(struct path *path)
> inode_unlock(path->dentry->d_inode);
> path_put(path);
> path->mnt = mnt;
> - dentry = path->dentry = dget(mnt->mnt_root);
> + path->dentry = dget(mnt->mnt_root);
> goto retry;
> }
>
> --
> 2.24.1
>
