On Fri, 26 Oct 2012, Oleg Nesterov wrote:
> On 10/26, Dave Chinner wrote: > > > > On Thu, Oct 25, 2012 at 10:09:31AM -0400, Mikulas Patocka wrote: > > > > > > Yes, mnt_want_write()/mnt_make_readonly() do the same thing as percpu rw > > > semaphores. I think you can convert mnt_want_write()/mnt_make_readonly() > > > to use percpu rw semaphores and remove the duplicated code. > > > > I think you misunderstood my point - that rather than re-inventing > > the wheel, why didn't you just copy something that is known to > > work? I didn't know about. The code is not reusable, and it doesn't really do locking. And it has two barriers on the read path, while percpu rw semaphores have none. > I don't understand why do you both think that __mnt_want_write() > and mnt_make_readonly() provides the same functionality. I looked > at this code before I started this patch, and unless I completely > misread it this does very different things. It is not "lock" at all. > > Oleg. mnt_want_write uses percpu array of counters, just like percpu semaphores. The code is different, but it can be changed to use percpu rw semaphores (if we add percpu_down_write_trylock). __mnt_want_write could call percpu_down_read and check if it is readonly (if it is, drop the lock and return -EROFS) __mnt_drop_write could call percpu_up_read mnt_make_readonly and sb_prepare_remount_readonly could call percpu_down_write_trylock instead of mnt_get_writers (if they get the write lock, set it to readonly and drop the write lock) ... and that's it, then, you can remove MNT_WRITE_HOLD, the barriers, spinning and other complexity from fs/namespace.c Mikulas -- 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/