Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()
From: Daniel J Blueman
Date: Tue Jan 14 2014 - 22:08:33 EST
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@xxxxxxxxxxxxx> 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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