Messages in this thread | | | Date | Wed, 27 Jan 2016 11:43:48 +0000 | From | Will Deacon <> | Subject | Re: [RFC][PATCH] mips: Fix arch_spin_unlock() |
| |
Hi Maciej,
Thanks for digging all this up.
On Wed, Jan 27, 2016 at 09:57:24AM +0000, Maciej W. Rozycki wrote: > On Thu, 12 Nov 2015, David Daney wrote: > > > > > Certainly we can load up the code with "SYNC" all over the place, but > > > > it will kill performance on SMP systems. So, my vote would be to make > > > > it as light weight as possible, but no lighter. That will mean > > > > inventing the proper barrier primitives. > > > > > > It seems to me that the proper barrier here is a "SYNC 18" aka > > > SYNC_RELEASE instruction, at least on CPUs that implement that variant. > > For the record, we've had "cooked" aliases in the toolchain for a short > while now -- since Sep 2010 or binutils 2.21 -- so for readability you can > actually use `sync_release' in your source code rather than obscure `sync > 18' (of course you could define a macro instead, but there's no need now), > and disassembly will show the "cooked" mnemonic too. > > Although Documentation/Changes still lists binutils 2.12 as the minimum, > so perhaps using macros is indeed the way to go now, at least for the time > being. > > > Yes, unfortunately very few CPUs implement that. It is an instruction that > > MIPS invented only recently, so older CPUs need a different solution. > > Hmm, it looks to me we might actually be safe, although as often the > situation seems more complicated than it had to be.
[... trim ISA archaeology ...]
> Overall I think it should be safe after all to use SYNC_RELEASE and other > modern lightweight barriers uncondtionally under the assumption that > architecture was meant to remain backward compatible. Even though it > might be possible someone would implement unusual semantics for the then > undefined `stype' values, I highly doubt it as it would be extra effort > and hardware logic space for no gain. We could try and reach architecture > overseers to double-check whether the `stype' encodings, somewhat > irregularly distributed, were indeed defined in a manner so as not to > clash with values implementers chose to use before rev. 2.61 of the > architecture specification.
Do you know whether a SYNC 18 (RELEASE) followed in program order by a SYNC 17 (ACQUIRE) creates a full barrier (i.e. something like SYNC 16)?
If not, you may need to implement smp_mb__after_unlock_lock for RCU to ensure globally transitive unlock->lock ordering should you decide to relax your locking barriers.
Will
| |