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From: Will Deacon <will@kernel.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Shaokun Zhang <zhangshaokun@hisilicon.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Yuqi Jin <jinyuqi@huawei.com>,
	kernel test robot <rong.a.chen@intel.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [NAK] Re: [PATCH] fs: Optimized fget to improve performance
Date: Fri, 28 Aug 2020 12:04:08 +0100	[thread overview]
Message-ID: <20200828110408.GA30722@willie-the-truck> (raw)
In-Reply-To: <20200827142848.GZ1236603@ZenIV.linux.org.uk>

On Thu, Aug 27, 2020 at 03:28:48PM +0100, Al Viro wrote:
> On Thu, Aug 27, 2020 at 06:19:44PM +0800, Shaokun Zhang wrote:
> > From: Yuqi Jin <jinyuqi@huawei.com>
> > 
> > It is well known that the performance of atomic_add is better than that of
> > atomic_cmpxchg.
> > The initial value of @f_count is 1. While @f_count is increased by 1 in
> > __fget_files, it will go through three phases: > 0, < 0, and = 0. When the
> > fixed value 0 is used as the condition for terminating the increase of 1,
> > only atomic_cmpxchg can be used. When we use < 0 as the condition for
> > stopping plus 1, we can use atomic_add to obtain better performance.
> 
> Suppose another thread has just removed it from the descriptor table.
> 
> > +static inline bool get_file_unless_negative(atomic_long_t *v, long a)
> > +{
> > +	long c = atomic_long_read(v);
> > +
> > +	if (c <= 0)
> > +		return 0;
> 
> Still 1.  Now the other thread has gotten to dropping the last reference,
> decremented counter to zero and committed to freeing the struct file.
> 
> > +
> > +	return atomic_long_add_return(a, v) - 1;
> 
> ... and you increment that sucker back to 1.  Sure, you return 0, so the
> caller does nothing to that struct file.  Which includes undoing the
> changes to its refecount.
> 
> In the meanwhile, the third thread does fget on the same descriptor,
> and there we end up bumping the refcount to 2 and succeeding.  Which
> leaves the caller with reference to already doomed struct file...
> 
> 	IOW, NAK - this is completely broken.  The whole point of
> atomic_long_add_unless() is that the check and conditional increment
> are atomic.  Together.  That's what your optimization takes out.

Cheers Al, yes, this is fscked.

As an aside, I've previously toyed with the idea of implementing a form
of cmpxchg() using a pair of xchg() operations and a smp_cond_load_relaxed(),
where the thing would transition through a "reserved value", which might
perform better with the current trend of building hardware that doesn't
handle CAS failure so well.

But I've never had the time/motivation to hack it up, and it relies on that
reserved value which obviously doesn't always work (so it would have to be a
separate API).

Will

  reply	other threads:[~2020-08-28 11:04 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-27 10:19 Shaokun Zhang
2020-08-27 12:30 ` Matthew Wilcox
2020-08-27 13:07   ` David Laight
2020-08-27 14:28 ` [NAK] " Al Viro
2020-08-28 11:04   ` Will Deacon [this message]
2020-08-31  1:43   ` Shaokun Zhang
2020-08-31  3:21     ` Al Viro
2020-09-01  9:29       ` David Laight

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