On Tue, May 06, 2008 at 04:29:13PM +0100, David Howells wrote: > Nick Piggin <npiggin@xxxxxxx> wrote: > > > While considering the impact of read_barrier_depends, it occurred to > > me that it should really be really a noop for the compiler. > > If you're defining it so, then you need to adjust memory-barriers.txt too. _I'm_ not defining it so, it has always been defined so (code speaks louder than words). So this document has always been wrong.i > > ======================== > EXPLICIT KERNEL BARRIERS > ======================== > ... > CPU MEMORY BARRIERS > ------------------- > > The Linux kernel has eight basic CPU memory barriers: > > TYPE MANDATORY SMP CONDITIONAL > =============== ======================= =========================== > GENERAL mb() smp_mb() > WRITE wmb() smp_wmb() > READ rmb() smp_rmb() > DATA DEPENDENCY read_barrier_depends() smp_read_barrier_depends() > > > All CPU memory barriers unconditionally imply compiler barriers. > > That last line needs modification, perhaps to say: > > General, read and write memory barriers unconditionally imply general > compiler barriers; data dependency barriers, however, imply a barrier > only for the specific access being performed due to the fact that the > instructions must be performed in a specific order. > > David -- 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
