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
