On Mon, Dec 09, 2013 at 09:26:41PM -0800, Paul E. McKenney wrote: > On Mon, Dec 09, 2013 at 05:34:17PM -0800, Josh Triplett wrote: > > On Mon, Dec 09, 2013 at 05:28:02PM -0800, Paul E. McKenney wrote: > > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> > > > > > > The Linux kernel has traditionally required that an UNLOCK+LOCK pair > > > act as a full memory barrier when either (1) that UNLOCK+LOCK pair > > > was executed by the same CPU or task, or (2) the same lock variable > > > was used for the UNLOCK and LOCK. It now seems likely that very few > > > places in the kernel rely on this full-memory-barrier semantic, and > > > with the advent of queued locks, providing this semantic either requires > > > complex reasoning, or for some architectures, added overhead. > > > > > > This commit therefore adds a smp_mb__after_unlock_lock(), which may be > > > placed after a LOCK primitive to restore the full-memory-barrier semantic. > > > All definitions are currently no-ops, but will be upgraded for some > > > architectures when queued locks arrive. > > > > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > > > Cc: Linux-Arch <linux-arch@xxxxxxxxxxxxxxx> > > > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > > > Cc: Oleg Nesterov <oleg@xxxxxxxxxx> > > > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > > > > It seems quite unfortunate that this isn't in some common location, and > > then only overridden by architectures that need to do so. > > I was thinking that include/asm-generic/barrier.h was the place, but > it is all-or-nothing, used by UP architectures, from what I can see. > I figured that if there is such a common location, posting this patch > might flush it out. I am not sure that this single definition is worth > the creation of a common place -- or even this definition combined with > smp_read_barrier_depends(). And of course the right place to put this is include/linux/spinlock.h, the same place where smp_mb__before_spinlock() is defined. Exceptions then go into the corresponding arch-specific spinlock.h files. Much better that way, thank you for calling this out! Thanx, Paul > > More importantly: you document this earlier in the patch series than you > > introduce it. > > Fair point, I reversed the order of those two patches. > > Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
