On Thu, 3 Apr 2008, Erez Zadok wrote: > In message <20080403182001.GB30189@xxxxxxxxxxxxxx>, "Josef 'Jeff' Sipek" writes: > > I think you need to check CONFIG_PREEMPT as well. > > I'm not sure if it's needed in case of CONFIG_PREEMPT. Anyone? The code > for i_size_write (below), and the comment at the top of the function, > suggest that the spinlock is needed only to prevent the lots seqcount. Correct. > BTW, some time ago I reviewed all callers of i_size_write. I did so again > just now, and the results were the same: > > - a LOT of callers of i_size_write don't take any lock They mostly know that i_mutex is already held (as i_size_write comment mentions); but I believe that's up to the individual filesystem. > - some take another spinlock in a different data structure > - those that do take the spinlock, do so unconditionally > - only unionfs and fs/stack.c wrap the spinlock in > > #if BITS_PER_LONG == 32 && defined(CONFIG_SMP) I chose to follow the #ifdeffery of i_size_write(), but you could do it unconditionally if you prefer: just a little more overhead when it's not needed. As I've said elsewhere, I don't think the result can be entirely safe against concurrent changes in the lower filesystem, using different locking; but I don't know how resilient unionfs is expected to be against messing directly with lower at the same time as upper level. Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
