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Date: Tue, 06 Jun 2017 14:25:09 -0400
From: Jeff Layton <jlayton@...hat.com>
To: Benjamin Coddington <bcodding@...hat.com>, bfields@...ldses.org,
Alexander Viro <viro@...iv.linux.org.uk>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] fs/locks: Remove fl_nspid
On Tue, 2017-06-06 at 14:00 -0400, Jeff Layton wrote:
> On Tue, 2017-06-06 at 13:19 -0400, Benjamin Coddington wrote:
> > Since commit c69899a17ca4 "NFSv4: Update of VFS byte range lock must be
> > atomic with the stateid update", NFSv4 has been inserting locks in rpciod
> > worker context. The result is that the file_lock's fl_nspid is the
> > kworker's pid instead of the original userspace pid.
> >
> > The fl_nspid is only used to represent the namespaced virtual pid number
> > when displaying locks or returning from F_GETLK. There's no reason to set
> > it for every inserted lock, since we can usually just look it up from
> > fl_pid. So, instead of looking up and holding struct pid for every lock,
> > let's just look up the virtual pid number from fl_pid when it is needed.
> > That means we can remove fl_nspid entirely.
> >
>
> With this set, I think we ought to codify that the stored pid must be
> relative
...to the init_pid_ns. Let's make that clear in the comments for
filesystem authors.
>
>
> > Signed-off-by: Benjamin Coddington <bcodding@...hat.com>
> > ---
> > fs/locks.c | 58 ++++++++++++++++++++++++++++++++----------------------
> > include/linux/fs.h | 1 -
> > 2 files changed, 35 insertions(+), 24 deletions(-)
> >
> > diff --git a/fs/locks.c b/fs/locks.c
> > index d7daa6c8932f..104398ccc9b9 100644
> > --- a/fs/locks.c
> > +++ b/fs/locks.c
> > @@ -733,7 +733,6 @@ static void locks_wake_up_blocks(struct file_lock *blocker)
> > static void
> > locks_insert_lock_ctx(struct file_lock *fl, struct list_head *before)
> > {
> > - fl->fl_nspid = get_pid(task_tgid(current));
> > list_add_tail(&fl->fl_list, before);
> > locks_insert_global_locks(fl);
> > }
> > @@ -743,10 +742,6 @@ locks_unlink_lock_ctx(struct file_lock *fl)
> > {
> > locks_delete_global_locks(fl);
> > list_del_init(&fl->fl_list);
> > - if (fl->fl_nspid) {
> > - put_pid(fl->fl_nspid);
> > - fl->fl_nspid = NULL;
> > - }
> > locks_wake_up_blocks(fl);
> > }
> >
> > @@ -823,8 +818,6 @@ posix_test_lock(struct file *filp, struct file_lock *fl)
> > list_for_each_entry(cfl, &ctx->flc_posix, fl_list) {
> > if (posix_locks_conflict(fl, cfl)) {
> > locks_copy_conflock(fl, cfl);
> > - if (cfl->fl_nspid)
> > - fl->fl_pid = pid_vnr(cfl->fl_nspid);
> > goto out;
> > }
> > }
> > @@ -2048,6 +2041,31 @@ int vfs_test_lock(struct file *filp, struct file_lock *fl)
> > }
> > EXPORT_SYMBOL_GPL(vfs_test_lock);
> >
> > +/**
> > + * locks_translate_pid - translate a pid number into a namespace
> > + * @nr: The pid number in the init_pid_ns
> > + * @ns: The namespace into which the pid should be translated
> > + *
> > + * Used to tranlate a fl_pid into a namespace virtual pid number
> > + */
> > +static pid_t locks_translate_pid(int init_nr, struct pid_namespace *ns)
> > +{
> > + pid_t vnr = 0;
> > + struct task_struct *task;
> > +
> > + rcu_read_lock();
> > + task = find_task_by_pid_ns(init_nr, &init_pid_ns);
> > + if (task)
> > + get_task_struct(task);
> > + rcu_read_unlock();
>
> Is that safe? What prevents get_task_struct from doing a 0->1 transition
> there after the task usage count has already gone 1->0 and is on its way
> to being freed?
>
> > + if (!task)
> > + goto out;
> > + vnr = task_pid_nr_ns(task, ns);
> > + put_task_struct(task);
> > +out:
> > + return vnr;
> > +}
Now that I look, I think it might be best to just do all of that under
the rcu_read_lock and don't muck with the refcount at all. So something
like:
+static pid_t locks_translate_pid(int init_nr, struct pid_namespace *ns)
+{
+ pid_t vnr = 0;
+ struct task_struct *task;
+
+ rcu_read_lock();
+ task = find_task_by_pid_ns(init_nr, &init_pid_ns);
+ if (task)
+ vnr = task_pid_nr_ns(task, ns);
+ rcu_read_unlock();
+ return vnr;
+}
That should be fine since task_pid_nr_ns pretty much does all of its
work under the rcu_read_lock anyway.
> > static int posix_lock_to_flock(struct flock *flock, struct file_lock *fl)
> > {
> > flock->l_pid = IS_OFDLCK(fl) ? -1 : fl->fl_pid;
> > @@ -2584,22 +2602,16 @@ static void lock_get_status(struct seq_file *f, struct file_lock *fl,
> > {
> > struct inode *inode = NULL;
> > unsigned int fl_pid;
> > + struct pid_namespace *proc_pidns = file_inode(f->file)->i_sb->s_fs_info;
> >
> > - if (fl->fl_nspid) {
> > - struct pid_namespace *proc_pidns = file_inode(f->file)->i_sb->s_fs_info;
> > -
> > - /* Don't let fl_pid change based on who is reading the file */
> > - fl_pid = pid_nr_ns(fl->fl_nspid, proc_pidns);
> > -
> > - /*
> > - * If there isn't a fl_pid don't display who is waiting on
> > - * the lock if we are called from locks_show, or if we are
> > - * called from __show_fd_info - skip lock entirely
> > - */
> > - if (fl_pid == 0)
> > - return;
> > - } else
> > - fl_pid = fl->fl_pid;
> > + fl_pid = locks_translate_pid(fl->fl_pid, proc_pidns);
> > + /*
> > + * If there isn't a fl_pid don't display who is waiting on
> > + * the lock if we are called from locks_show, or if we are
> > + * called from __show_fd_info - skip lock entirely
> > + */
> > + if (fl_pid == 0)
> > + return;
> >
> > if (fl->fl_file != NULL)
> > inode = locks_inode(fl->fl_file);
> > @@ -2674,7 +2686,7 @@ static int locks_show(struct seq_file *f, void *v)
> >
> > fl = hlist_entry(v, struct file_lock, fl_link);
> >
> > - if (fl->fl_nspid && !pid_nr_ns(fl->fl_nspid, proc_pidns))
> > + if (locks_translate_pid(fl->fl_pid, proc_pidns) == 0)
> > return 0;
> >
> > lock_get_status(f, fl, iter->li_pos, "");
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index aa4affb38c39..b013fac515f7 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -984,7 +984,6 @@ struct file_lock {
> > unsigned char fl_type;
> > unsigned int fl_pid;
> > int fl_link_cpu; /* what cpu's list is this on? */
> > - struct pid *fl_nspid;
> > wait_queue_head_t fl_wait;
> > struct file *fl_file;
> > loff_t fl_start;
>
>
--
Jeff Layton <jlayton@...hat.com>
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