linux-kernel - Re: [PATCH] futex: Reduce the scope of lock_page, aka lockless futex_get

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Message-ID: <20160105204342.GA15465@linux-uzut.site>
Date:	Tue, 5 Jan 2016 12:43:42 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mel Gorman <mgorman@...hsingularity.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Chris Mason <clm@...com>, Darren Hart <dvhart@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH] futex: Reduce the scope of lock_page, aka lockless
 futex_get_key()

On Tue, 05 Jan 2016, Peter Zijlstra wrote:

>On Tue, Jan 05, 2016 at 12:23:55PM -0800, Davidlohr Bueso wrote:
>> +	if (unlikely(!mapping)) {
>> +		int shmem_swizzled;
>> +
>> +		/*
>> +		 * Page lock is required to identify which special case above
>> +		 * applies. If this is really a shmem page then the page lock
>> +		 * will prevent unexpected transitions.
>> +		 */
>> +		lock_page(page);
>> +		shmem_swizzled = PageSwapCache(page);
>>  		unlock_page(page);
>>  		put_page(page);
>> +		WARN_ON_ONCE(mapping);
>
>We've not re-loaded mapping, so how could this possibly be?

Yep, this wants to be page->mapping.

>
>
>> +		/*
>> +		 * Take a reference unless it is about to be freed. Previously
>> +		 * this reference was taken by ihold under the page lock
>> +		 * pinning the inode in place so i_lock was unnecessary. The
>> +		 * only way for this check to fail is if the inode was
>> +		 * truncated in parallel so warn for now if this happens.
>> +		 *
>> +		 * TODO: VFS and/or filesystem people should review this check
>> +		 * and see if there is a safer or more reliable way to do this.
>> +		 */
>> +		if (WARN_ON(!atomic_inc_not_zero(&inode->i_count))) {
>> +			rcu_read_unlock();
>> +			put_page(page);
>> +			goto again;
>> +		}
>> +
>> +		/*
>> +		 * get_futex_key() must imply MB (B) and we are not going to
>> +		 * call into get_futex_key_refs() at this point.
>> +		 */
>> +		smp_mb__after_atomic();
>
>I don't get this one, the above is a successful atomic op with return
>value, that _must_ imply a full barrier.

Ah sure, I was actually following convention of what we have for our plain atomic_inc,
but in this case we are returning a value, so yeah, it is not required. Will drop.

Thanks,
Davidlohr

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