On Wed, Aug 12, 2009 at 11:15 AM, shailesh
jain<coolworldofshail@xxxxxxxxx> wrote:
> I haven't looked the code but ...
> You can unlock in any order, right ? .... because whatever happens you will
> always be able to unlock and it the locks are acquired in the same order
> throughout the system, then your system is guaranteed to make forward
> progress.
Hmmm ok ... "same order throughout" is the key.
Thanks -
Manish
>
> As an exercise try coming up with an example that leads to you deadlock when
> you unlock in incorrect order.
>
>
> Shailesh Jain
>
>
> On Wed, Aug 12, 2009 at 12:32 AM, Manish Katiyar <mkatiyar@xxxxxxxxx> wrote:
>>
>> Hi,
>>
>> I was going through this code below and was confused why do we need
>> lock ordering here. What is the problem that it is trying to prevent
>> given that the corresponding unlocking doesn't care for the order. I
>> assume lock orders are primarily needed to avoid deadlocks, and if
>> they are then they need to be unlocked in the same order too.
>>
>> File : fs/ext4/move_extent.c
>>
>> /**
>> * mext_double_down_write - Acquire two inodes' write semaphore
>> *
>> * @orig_inode: original inode structure
>> * @donor_inode: donor inode structure
>> * Acquire write semaphore of the two inodes (orig and donor) by i_ino
>> order.
>> */
>> static void
>> mext_double_down_write(struct inode *orig_inode, struct inode
>> *donor_inode)
>> {
>> struct inode *first = orig_inode, *second = donor_inode;
>>
>> BUG_ON(orig_inode == NULL || donor_inode == NULL);
>>
>> /*
>> * Use the inode number to provide the stable locking order instead
>> * of its address, because the C language doesn't guarantee you can
>> * compare pointers that don't come from the same array.
>> */
>> if (donor_inode->i_ino < orig_inode->i_ino) {
>> first = donor_inode;
>> second = orig_inode;
>> }
>>
>> down_write(&EXT4_I(first)->i_data_sem);
>> down_write(&EXT4_I(second)->i_data_sem);
>> }
>>
>> /**
>> * mext_double_up_read - Release two inodes' read semaphore
>> *
>> * @orig_inode: original inode structure to be released its lock
>> first
>> * @donor_inode: donor inode structure to be released its lock
>> second
>> * Release read semaphore of two inodes (orig and donor).
>> */
>> static void
>> mext_double_up_read(struct inode *orig_inode, struct inode *donor_inode)
>> {
>> BUG_ON(orig_inode == NULL || donor_inode == NULL);
>>
>> up_read(&EXT4_I(orig_inode)->i_data_sem);
>> up_read(&EXT4_I(donor_inode)->i_data_sem);
>> }
>>
>>
>> --
>> Thanks -
>> Manish
>>
>> --
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>>
>
>
--
Thanks -
Manish
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