Re: [RFC] Re: broken userland ABI in configfs binary attributes

[Miklos Szeredi <miklos@szeredi.hu>]

On Mon, Aug 26, 2019 at 08:28:19PM +0100, Al Viro wrote:
> On Mon, Aug 26, 2019 at 11:20:17AM -0700, Matthew Wilcox wrote:
> > On Mon, Aug 26, 2019 at 05:29:49PM +0100, Al Viro wrote:
> > > On Mon, Aug 26, 2019 at 03:48:38AM +0100, Al Viro wrote:
> > > 
> > > > 	We might be able to paper over that mess by doing what /dev/st does -
> > > > checking that file_count(file) == 1 in ->flush() instance and doing commit
> > > > there in such case.  It's not entirely reliable, though, and it's definitely
> > > > not something I'd like to see spreading.
> > > 
> > > 	This "not entirely reliable" turns out to be an understatement.
> > > If you have /proc/*/fdinfo/* being read from at the time of final close(2),
> > > you'll get file_count(file) > 1 the last time ->flush() is called.  In other
> > > words, we'd get the data not committed at all.

How about something like this:

#if BITS_PER_LONG == 32
#define F_COUNT_SHORTTERM ((1UL << 24) + 1)
#else
#define F_COUNT_SHORTTERM ((1UL << 48) + 1)
#endif

static inline void get_file_shortterm(struct file *f)
{
	atomic_long_add(F_COUNT_SHORTTERM, &f->f_count);
}

static inline void put_file_shortterm(struct file *f)
{
	fput_many(f, F_COUNT_SHORTTERM);
}

static inline bool file_is_last_longterm(struct file *f)
{
	return atomic_long_read(&f->f_count) % F_COUNT_SHORTTERM == 1;
}

Thanks,
Miklos