linux-kernel - Re: [PATCH] [mmotm] comment on swap notify locking constraints

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Date:	Thu, 14 Jan 2010 23:29:47 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
CC:	Pekka Enberg <penberg@...helsinki.fi>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...IV.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Andi Kleen <andi@...stfloor.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [mmotm] comment on swap notify locking constraints

On 01/14/2010 08:40 PM, Hugh Dickins wrote:
> On Thu, 14 Jan 2010, Nitin Gupta wrote:
> 
>> A block device makes swap_slot_free_notify() callback
>> when the last reference to a swap slot is dropped.
>>
>> This callback is made under swap_lock and often with
>> lock for corresponding swapcache page also held. This
>> is a note of warning for registered callback function 
>> which must meet these constraints.
> 
> You're right that the page lock of the corresponding swapcache page
> may be held there, but that isn't what worried me enough to ask for
> a comment: it's the page table lock (sometimes we call it pte lock),
> a spinlock like the swap_lock, being held there that particularly
> deserves comment.  We don't want someone going to lots of effort
> to avoid holding swap_lock there, only to find that effort wasted
> because page table lock is also held.

Thanks for the review.

I was missing this important point regarding pte lock. I will edit
the comment and resend the patch.

Nitin





>>
>> Signed-off-by: Nitin Gupta <ngupta@...are.org>
>> ---
>>  include/linux/blkdev.h |    1 +
>>  1 files changed, 1 insertions(+), 0 deletions(-)
>>
>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>> index 14b95a3..e3dcff6 100644
>> --- a/include/linux/blkdev.h
>> +++ b/include/linux/blkdev.h
>> @@ -1295,6 +1295,7 @@ struct block_device_operations {
>>  						unsigned long long);
>>  	int (*revalidate_disk) (struct gendisk *);
>>  	int (*getgeo)(struct block_device *, struct hd_geometry *);
>> +	/* this callback is with swap_lock and often page lock also held */
> 
>                                                      and page table lock
> 
>>  	void (*swap_slot_free_notify) (struct block_device *, unsigned long);
>>  	struct module *owner;
>>  };
>> -- 
>> 1.6.2.5
> 

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