Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd_install

[Mateusz Guzik <mguzik@redhat.com>]

On Tue, Apr 21, 2015 at 10:06:24PM +0200, Mateusz Guzik wrote:
> On Tue, Apr 21, 2015 at 11:05:43AM -0700, Eric Dumazet wrote:
> > On Mon, 2015-04-20 at 13:49 -0700, Eric Dumazet wrote:
> > > On Mon, 2015-04-20 at 10:15 -0700, Eric Dumazet wrote:
> > > > On Mon, 2015-04-20 at 17:10 +0200, Mateusz Guzik wrote:
> > > > 
> > > > > Sorry for spam but I came up with another hack. :)
> > > > > 
> > > > > The idea is that we can have a variable which would signify the that
> > > > > given thread is playing with fd table in fd_install (kind of a lock
> > > > > embedded into task_struct). We would also have a flag in files struct
> > > > > indicating that a thread would like to resize it.
> > > > > 
> > > > > expand_fdtable would set the flag and iterate over all threads waiting
> > > > > for all of them to have the var set to 0.
> > > > 
> > > > The opposite : you have to block them in some way and add a rcu_sched()
> > > > or something.
> > > 
> 
> What I described would block them, although it was a crappy approach
> (iterating threads vs cpus). I was wondering if RCU could be abused for
> this feature and apparently it can.
> 
> > > Here is the patch I cooked here but not yet tested.
> > 
> > In following version :
> > 
> > 1) I replaced the yield() hack by a proper wait queue.
> > 
> > 2) I do not invoke synchronize_sched() for mono threaded programs.
> > 
> > 3) I avoid multiple threads doing a resize and then only one wins the
> > deal.
> > 
> 
> One could argue this last bit could be committed separately (a different
> logical change).
> 
> As I read up about synchronize_sched and rcu_read_lock_sched, the code
> should be correct.
> 
> Also see nits below.
> 

I'm utter crap with memory barriers, but I believe some are needed now:

in dup_fd:
       for (i = open_files; i != 0; i--) {
                struct file *f = *old_fds++;
                if (f) {
                        get_file(f);

at least a data dependency barrier, or maybe smp_rmb for peace of mind

similarly in do_dup2:
        tofree = fdt->fd[fd];
        if (!tofree && fd_is_open(fd, fdt))
                goto Ebusy;

> > (copying/clearing big amount of memory only to discover another guy did
> > the same is a big waste of resources)
> > 
> > 
> > This seems to run properly on my hosts.
> > 
> > Your comments/tests are most welcomed, thanks !
> > 
> >  fs/file.c               |   41 +++++++++++++++++++++++++++++++++-----
> >  include/linux/fdtable.h |    3 ++
> >  2 files changed, 39 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/file.c b/fs/file.c
> > index 93c5f89c248b..e0e113a56444 100644
> > --- a/fs/file.c
> > +++ b/fs/file.c
> > @@ -147,6 +147,9 @@ static int expand_fdtable(struct files_struct *files, int nr)
> >  
> >  	spin_unlock(&files->file_lock);
> >  	new_fdt = alloc_fdtable(nr);
> > +	/* make sure no __fd_install() are still updating fdt */
> > +	if (atomic_read(&files->count) > 1)
> > +		synchronize_sched();
> >  	spin_lock(&files->file_lock);
> >  	if (!new_fdt)
> >  		return -ENOMEM;
> > @@ -170,9 +173,12 @@ static int expand_fdtable(struct files_struct *files, int nr)
> >  		if (cur_fdt != &files->fdtab)
> >  			call_rcu(&cur_fdt->rcu, free_fdtable_rcu);
> >  	} else {
> > +		WARN_ON_ONCE(1);
> >  		/* Somebody else expanded, so undo our attempt */
> >  		__free_fdtable(new_fdt);
> 
> The reader may be left confused why there is a warning while the comment
> does not indicate anything is wrong.
> 
> >  	}
> > +	/* coupled with smp_rmb() in __fd_install() */
> > +	smp_wmb();
> >  	return 1;
> >  }
> >  
> > @@ -187,19 +193,33 @@ static int expand_fdtable(struct files_struct *files, int nr)
> >  static int expand_files(struct files_struct *files, int nr)
> >  {
> >  	struct fdtable *fdt;
> > +	int expanded = 0;
> >  
> > +begin:
> >  	fdt = files_fdtable(files);
> >  
> >  	/* Do we need to expand? */
> >  	if (nr < fdt->max_fds)
> > -		return 0;
> > +		return expanded;
> >  
> >  	/* Can we expand? */
> >  	if (nr >= sysctl_nr_open)
> >  		return -EMFILE;
> >  
> > +	while (unlikely(files->resize_in_progress)) {
> > +		spin_unlock(&files->file_lock);
> > +		expanded = 1;
> > +		wait_event(files->resize_wait, !files->resize_in_progress);
> > +		spin_lock(&files->file_lock);
> > +		goto begin;
> > +	}
> 
> This does not loop anymore, so s/while/if/ ?
> 
> > +
> >  	/* All good, so we try */
> > -	return expand_fdtable(files, nr);
> > +	files->resize_in_progress = true;
> > +	expanded = expand_fdtable(files, nr);
> > +	files->resize_in_progress = false;
> > +	wake_up_all(&files->resize_wait);
> > +	return expanded;
> >  }
> >  
> >  static inline void __set_close_on_exec(int fd, struct fdtable *fdt)
> > @@ -256,6 +276,8 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
> >  	atomic_set(&newf->count, 1);
> >  
> >  	spin_lock_init(&newf->file_lock);
> > +	newf->resize_in_progress = 0;
> > +	init_waitqueue_head(&newf->resize_wait);
> >  	newf->next_fd = 0;
> >  	new_fdt = &newf->fdtab;
> >  	new_fdt->max_fds = NR_OPEN_DEFAULT;
> > @@ -553,11 +575,20 @@ void __fd_install(struct files_struct *files, unsigned int fd,
> >  		struct file *file)
> >  {
> >  	struct fdtable *fdt;
> > -	spin_lock(&files->file_lock);
> > -	fdt = files_fdtable(files);
> > +
> > +	rcu_read_lock_sched();
> > +
> > +	while (unlikely(files->resize_in_progress)) {
> > +		rcu_read_unlock_sched();
> > +		wait_event(files->resize_wait, !files->resize_in_progress);
> > +		rcu_read_lock_sched();
> > +	}
> > +	/* coupled with smp_wmb() in expand_fdtable() */
> > +	smp_rmb();
> > +	fdt = READ_ONCE(files->fdt);
> >  	BUG_ON(fdt->fd[fd] != NULL);
> >  	rcu_assign_pointer(fdt->fd[fd], file);
> > -	spin_unlock(&files->file_lock);
> > +	rcu_read_unlock_sched();
> >  }
> >  
> >  void fd_install(unsigned int fd, struct file *file)
> > diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
> > index 230f87bdf5ad..fbb88740634a 100644
> > --- a/include/linux/fdtable.h
> > +++ b/include/linux/fdtable.h
> > @@ -47,6 +47,9 @@ struct files_struct {
> >     * read mostly part
> >     */
> >  	atomic_t count;
> > +	bool resize_in_progress;
> > +	wait_queue_head_t resize_wait;
> > +
> >  	struct fdtable __rcu *fdt;
> >  	struct fdtable fdtab;
> >    /*
> > 
> > 
> > 
> > 
> 
> -- 
> Mateusz Guzik

-- 
Mateusz Guzik