Messages in this thread | | | Date | Wed, 15 Jan 2014 11:08:16 +0800 | From | Daniel J Blueman <> | Subject | Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock() |
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On 01/15/2014 07:44 AM, Paul E. McKenney wrote: > On Tue, Jan 14, 2014 at 10:01:04AM -0800, Richard Henderson wrote: >> On 01/14/2014 09:08 AM, Matt Turner wrote: >>> On Tue, Jan 14, 2014 at 3:03 AM, Peter Zijlstra <peterz@infradead.org> wrote: >>>> On Tue, Jan 14, 2014 at 10:28:23AM +0800, Daniel J Blueman wrote: >>>>>> Peter, >>>>>> >>>>>> I found out that the build failure was caused by the fact that the >>>>>> __native_word() macro (used internally by compiletime_assert_atomic()) >>>>>> allows only a size of 4 or 8 for x86-64. The data type that I used is a >>>>>> byte. Is there a reason why byte and short are not considered native? >>>>> >>>>> It seems likely it was implemented like that since there was no existing >>>>> need; long can be relied on as the largest native type, so this should >>>>> suffice and works here: >>>> >>>> There's Alphas that cannot actually atomically adres a byte; I do not >>>> konw if Linux cares about them, but if it does, we cannot in fact rely >>>> on this in generic primitives like this. >>> >>> That's right, and thanks for the heads-up. Alpha can only address 4 >>> and 8 bytes atomically. (LDL_L, LDQ_L, STL_C, STQ_C). >>> >>> The Byte-Word extension in EV56 doesn't add new atomics, so in fact no >>> Alphas can address < 4 bytes atomically. >> >> Emulated with aligned 4 byte atomics, and masking. The same is true for arm, >> ppc, mips which, depending on cpu, also lack < 4 byte atomics. > > Which means that Alpha should be able to similarly emulate 1-byte and > 2-byte atomics, correct?
If it's not possible to guarantee that GCC emits the 4-byte atomics by using a union, then we could emit the instructions via assembly. We'd introduce a macro to ensure lock word alignment, and this would be safe for unsigned counting up to the packed type limit. Maybe that's just too over-constrained though.
Thanks, Daniel -- Daniel J Blueman Principal Software Engineer, Numascale
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