On 04/02/2014 11:41 AM, Jan Kara wrote: > Thanks for the patches and measurements! So I agree we contend a lot on > orphan list changes in ext4. But what you do seems to be unnecessarily > complicated and somewhat hiding the real substance of the patch. If I > understand your patch correctly, all it does is that it does the > preliminary work (ext4_reserve_inode_write(), > ext4_journal_get_write_access()) without the global orphan mutex (under the > hashed mutex). >
Thanks Jan for the comments. Yes, doing some of the preliminary work with grabbing the global mutex is part of the patch's strategy. > However orphan operations on a single inode are already serialized by > i_mutex so there's no need to introduce any new hashed lock. Just add > assertion mutex_locked(&inode->i_mutex) to ext4_orphan_add() and > ext4_orphan_del() - you might need to lock i_mutex around the code in > fs/ext4/migrate.c and in ext4_tmpfile() but that should be fine. > As you pointed out, sounds like there may still be some code path that did not acquire the i_mutex. It probably would be better to acquire the i_mutex if it is not already acquired. > Also I'm somewhat failing to see what the spinlock s_orphan_lock brings us. > I'd guess that the mutex could still protect also the in-memory list and we > have to grab it in all the relevant cases anyway (in some rare cases we > could avoid taking the mutex and spinlock would be enough but these > shouldn't be performance relevant). Please correct me if I'm wrong here, I > didn't look at the code for that long. > Yes, you are correct. In the error or previous error case, we only need to update the in memory orphan list, which spinlock seems to be a better mechanism for serialization. Using a separate spinlock would also allow simultanoue operations of both the error and non-error cases. As you said, if this is a very rare case, it should not make much different. I'll rerun and ompare the benchmark results using a single mutex. > Finally (and I somewhat miss this in your patch), I'd think we might need > to use list_empty_careful() when checking inode's orphan list without > global orphan list lock. > > Honza > Since we already serialize orphan operation with a single inode, the only race condition is an orphan operation on other inodes moving the inode within the orphan list. In this case head->next should not equal head. But yes, it is probably safer to use the list_empty_careful(). Thanks, Mak. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
