On Sun, 2008-06-29 at 14:55 -0600, Grant Grundler wrote: > On Sat, Jun 28, 2008 at 05:34:07PM -0600, Matthew Wilcox wrote: > > On Sat, Jun 28, 2008 at 06:07:57PM -0400, John David Anglin wrote: > > > There are two reasons to expose the memory *a in the asm: > > > > > > 1) To prevent the compiler from discarding a preceeding write to *a, and > > > 2) to prevent it from caching *a in a register over the asm. > > > > Do either of those scenarios apply, given that every usage of this is > > preceded by an asm clobbering memory? > > > > I believe the correct thing to do is to take out the two mb()s in the > > various spin_lock routines and make the __ldcw() macro itself clobber > > memory. > > I agree. Do you want jda to submit another patch or did you want kyle to > take jda's patch and apply a second one to remove the mb()'s? I really wouldn't do that. Parisc is fairly unique in that we have C spinlocks (most other architectures have asm coded ones). The requirement of the spinlock routines are that they be atomic memory clobbers (as in the sequence of statements that does one shouldn't be moved by the compiler)---which is why our C ones have mb before and after. if you make __ldcw() a memory clobber, that will pretty much cover the atomic memory clobber requirements of __raw_spin_lock_flags() ... but look at the rest of them ... it won't cover for them. If we have to have these mb()s in the rest, it makes sense to have them in __raw_spin_lock_flags() as well, just to avoid problems if someone tries to optimise again. There probably should be a comment in this file to that effect. James -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
