linux-ext4 - Re: Copying Data Blocks

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Date:	Mon, 12 Jan 2009 15:41:34 +0800
From:	Peter Teoh <htmldeveloper@...il.com>
To:	Rohit Sharma <imreckless@...il.com>
Cc:	ext4 <linux-ext4@...r.kernel.org>,
	Kernelnewbies <kernelnewbies@...linux.org>
Subject: Re: Copying Data Blocks

If u grep for spinlock, mutex, or "sem" in the fs/ext4 directory, u
can find all three types of lock are used - for different class of
object.

For data blocks I guessed is semaphore - read this
fs/ext4/inode.c:ext4_get_branch():

/**
 *      ext4_get_branch - read the chain of indirect blocks leading to data
<snip>
 *
 *      Need to be called with
 *      down_read(&EXT4_I(inode)->i_data_sem)
 */

i guess u have no choice, as it is semaphore, have to follow the rest
of kernel for consistency - don't create your own semaphore :-).

There exists i_lock as spinlock - which so far i know is for i_blocks
counting purposes:

       spin_lock(&inode->i_lock);
        inode->i_blocks += tmp_inode->i_blocks;
        spin_unlock(&inode->i_lock);
        up_write(&EXT4_I(inode)->i_data_sem);

But for data it should be i_data_sem.   Is that correct?

On Mon, Jan 12, 2009 at 2:18 PM, Rohit Sharma <imreckless@...il.com> wrote:
> Hi,
>
> I am having some issues in locking inode while copying data blocks.
> We are trying to keep file system live during this operation, so
> both read and write operations should work.
> In this case what type of lock on inode should be used, semaphore,
> mutex or spinlock?
>
>
> On Sun, Jan 11, 2009 at 8:45 PM, Peter Teoh <htmldeveloper@...il.com> wrote:
>> Sorry.....some mistakes...a resent:
>>
>> Here are some tips on the blockdevice API:
>>
>> http://lkml.org/lkml/2006/1/24/287
>> http://linux.derkeiler.com/Mailing-Lists/Kernel/2006-01/msg09388.html
>>
>> as indicated, documentation is rather sparse in this area.
>>
>> not sure if anyone else have a summary list of blockdevice API and its
>> explanation?
>>
>> not wrt the following "cleanup patch", i am not sure how the API will change:
>>
>> http://lwn.net/Articles/304485/
>>
>> thanks.
>>
>> On Tue, Jan 6, 2009 at 6:36 PM, Rohit Sharma <imreckless@...il.com> wrote:
>>>
>>> I want to read data blocks from one inode
>>> and copy it to other inode.
>>>
>>> I mean to copy data from data blocks associated with one inode
>>> to the data blocks associated with other inode.
>>>
>>> Is that possible in kernel space.?
>>> --
>>
>>
>>
>> --
>> Regards,
>> Peter Teoh
>>
>



-- 
Regards,
Peter Teoh
--
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