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.
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 <[email protected]> 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 > > -- > To unsubscribe from this list: send an email with > "unsubscribe kernelnewbies" to [email protected] > Please read the FAQ at http://kernelnewbies.org/FAQ > >
