Hello, On Tue, Nov 11, 2014 at 06:57:05PM +0000, alexander.duyck@xxxxxxxxx wrote: > From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxx> > > In the case of device drivers it is common to utilize receive descriptors > in which a single field is used to determine if the descriptor is currently > in the possession of the device or the CPU. In order to prevent any other > fields from being read a rmb() is used resulting in something like code > snippet from ixgbe_main.c: > > if (!ixgbe_test_staterr(rx_desc, IXGBE_RXD_STAT_DD)) > break; > > /* > * This memory barrier is needed to keep us from reading > * any other fields out of the rx_desc until we know the > * RXD_STAT_DD bit is set > */ > rmb(); > > On reviewing the documentation and code for smp_load_acquire() it occured > to me that implementing something similar for CPU <-> device interraction > would be worth while. This commit provides just the load/read side of this > in the form of read_acquire(). This new primative orders the specified > read against any subsequent reads. As a result we can reduce the above > code snippet down to: > > /* This memory barrier is needed to keep us from reading > * any other fields out of the rx_desc until we know the > * RXD_STAT_DD bit is set > */ > if (!(read_acquire(&rx_desc->wb.upper.status_error) & Minor nit on naming, but load_acquire would match what we do with barriers, where you simply drop the smp_ prefix if you want the thing to work on UP systems too. > cpu_to_le32(IXGBE_RXD_STAT_DD))) > break; I'm not familiar with the driver in question, but how are the descriptors mapped? Is the read barrier here purely limiting re-ordering of normal memory accesses by the CPU? If so, isn't there also scope for store_release when updating, e.g. next_to_watch in the same driver? We also need to understand how this plays out with smp_mb__after_unlock_lock, which is currently *only* implemented by PowerPC. If we end up having a similar mess to mmiowb, where PowerPC both implements the barrier *and* plays tricks in its spin_unlock code, then everybody loses because we'd end up with release doing the right thing anyway. Peter and I spoke with Paul at LPC about strengthening smp_load_acquire/smp_store_release so that release->acquire ordering is maintained, which would allow us to drop smp_mb__after_unlock_lock altogether. That's stronger than acquire/release in C11, but I think it's an awful lot easier to use, particularly if device drivers are going to start using these primitives. Thoughts? Will -- 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
