On Sun, Oct 24, 2010 at 05:44:24PM +0200, Thomas Gleixner wrote: > On Tue, 19 Oct 2010, Christoph Hellwig wrote: > > > On Tue, Oct 19, 2010 at 06:00:57PM +1100, Nick Piggin wrote: > > > But it is still "magic". Because you don't even know whether it > > > is a spin or sleeping lock, let alone whether it is irq or bh safe. > > > You get far more information seeing a bit_spin_lock(0, &hlist) call > > > than hlist_lock(). > > Errm, when hlist_lock() has proper documentation than it should not be > rocket science to figure out what it does. Right, a look at the docmentation and another layer of indirection for a reader. And it's not exactly "properly" documented. It doesn't say if it may turn into a sleeping lock or is allowed to be used from irq or bh context. > And if you use bit 0 of hlist then you better have helper functions to > access it anyway. We do that with other data types which (ab)use the > lower two bits of pointers. > > > To get back a bit to the point: > > > > - we have a new bl_hlist sturcture which combines a hash list and a > > lock embedded into the head > > - the reason why we do it is to be able to use a bitlock > > And if you design that structure clever, then simple dereferencing of > it (w/o casting magic) should make the compiler barf. So you are > forced to use the helper functions. > > > Furthermore it allows the RT people to simply throw a mutex into the > > head and everything keeps working without touching a sinlge line of > > code outside of hlist_bl.h. > > Yes, please use proper helper functions. Having to change code is a > horror for RT, when we can get away with a single change in a header > file. > > Aside of RT there is another advantage of being able to change the > lock implementation at a single place: you can change it to a real > spinlock and have lockdep coverage of that code. I fundamentally hate > bit_spin_locks for sneaking around lockdep. You do not want to add a bloated mutex to each inode hash bucket and think you can just dust off your hands and walk away. You would probably make a smaller auxiliary hash of locks, sanely sized, and protect it with that. So it would be wrong to just bloat hlist_bl by a factor of several times (how big is a mutex in -rt?) without doing anything else. Although a sane locking macro and structure like I had, would perfectly allow you to switch locks in a single place just the same. -- 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
