From: "Sandeep K Sinha" <sandeepksinha@gmail.com>
Subject: Re: Copying Data Blocks
Date: Mon, 12 Jan 2009 13:56:17 +0530
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Cc: "Rohit Sharma" <imreckless@gmail.com>,
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To: "Peter Teoh" <htmldeveloper@gmail.com>
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Hi Peter,

Don't you think that if will restrict this to a specific file system.
VFS inode should be used rather than the FS incore inode ?

The purpose if to sleep all the i/o's when we are updating the i_data
from the new inode to the old inode ( updation of the data blocks ).

I think i_alloc_sem should work here, but could not find any instance
of its use in the code.
It's working fine currently with i_mutex, meaning if we hold a i_mutex
lock on the inode while updating the i_data pointers.
And try to perform i/o from user space, they are queued. The file was
opened in r/w mode prior to taking the lock inside the kernel.

But, I still feel i_alloc_sem would be the right option to go ahead with.

On Mon, Jan 12, 2009 at 1:11 PM, Peter Teoh <htmldeveloper@gmail.com> wrote:
> 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@gmail.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@gmail.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@gmail.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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>



-- 
Regards,
Sandeep.





 	
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