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Date:   Tue, 27 Aug 2019 12:58:08 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Christoph Hellwig <hch@lst.de>, linux-fsdevel@vger.kernel.org,
        Octavian Purdila <octavian.purdila@intel.com>,
        Pantelis Antoniou <pantelis.antoniou@konsulko.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Kai =?iso-8859-1?Q?M=E4kisara?= <Kai.Makisara@kolumbus.fi>,
        linux-scsi@vger.kernel.org
Subject: Re: [RFC] Re: broken userland ABI in configfs binary attributes
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On Tue, Aug 27, 2019 at 10:51:44AM +0200, Miklos Szeredi wrote:
 
> 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;
> }

So 256 threads boinking on the same fdinfo at the same time
and struct file can be freed right under them?  Or a bit over
million of dup(), then forking 15 more children, for that matter...

Seriously, it might be OK on 64bit (with something like "no more
than one reference held by a thread", otherwise you'll run
into overflows even there - 65536 of your shortterm references
aren't that much).  On 32bit it's a non-starter - too easy to
overflow.