linux-kernel - Re: [PATCH v5] futex: Remove requirement for lock_page in get_futex

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Message-ID: <20160204174343.GA12375@linux-uzut.site>
Date:	Thu, 4 Feb 2016 09:43:43 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Mel Gorman <mgorman@...hsingularity.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Chris Mason <clm@...com>, Darren Hart <dvhart@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dbueso@...e.de>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v5] futex: Remove requirement for lock_page in
 get_futex_key

On Thu, 04 Feb 2016, Thomas Gleixner wrote:

>On Wed, 3 Feb 2016, Davidlohr Bueso wrote:
>> +		 * We are not calling into get_futex_key_refs() in file-backed
>> +		 * cases, therefore a successful atomic_inc return below will
>> +		 * guarantee that get_futex_key() will continue to imply MB (B).
>
>Can you please make that "MB (B)" part a bit more outstanding. I really had to
>search for it.

Hmm as you know this is mostly explained at the begining of the file, and we
sprinkle MB (B) around the code based on that description. So I'm a bit confused
as to why you don't like like that comment.

>> +		 */
>> +		if (WARN_ON_ONCE(!atomic_inc_not_zero(&inode->i_count))) {
>> +			rcu_read_unlock();
>> +			put_page(page);
>> +
>> +			goto again;
>> +		}
>
>Don't we need
>
>		smp_mb__after_atomic();
>
>here to make it a full barrier on all architectures?

I had this initially but, as Peter mentioned, we get that barrier with the
successful atomic_inc_not_zero call anyway. Or is it something else you had
in mind?

Thanks,
Davidlohr