I understand that InterlockedExchange64 works to implement a 64-bit
store. I also understand 32-bit builds use it for 64-bit atomic stores:
because 32-bit code cannot otherwise store to 64-bit objects. I don't
understand why 64-bit code would use it for 64-bit atomic stores; it
does not make any sense for MSVC to convert such a store to two 32-bit
instructions, and I doubt that it does.
Here's the code I'm talking about:
/* MSVC converts 64 bit writes into two instructions. So there is
* a possibility that an interrupt can make a 64 bit write non-atomic even
* when 8 byte aligned. So use InterlockedExchange64().
*
* For atomic stores, 'consume' and 'acquire' semantics are not valid. But we
* are using 'Exchange' to get atomic stores here and we only have
* InterlockedExchange64(), InterlockedExchangeNoFence64() and
* InterlockedExchange64Acquire() available. So we are forced to use
* InterlockedExchange64() which uses full memory barrier for everything
* greater than 'memory_order_relaxed'. */
#define atomic_store64(DST, SRC, ORDER) \
if (ORDER == memory_order_relaxed) { \
InterlockedExchangeNoFence64((int64_t volatile *) (DST), \
(int64_t) (SRC)); \
} else { \
InterlockedExchange64((int64_t volatile *) (DST), (int64_t) (SRC));\
}
Similarly for atomic_read64():
/* MSVC converts 64 bit reads into two instructions. So there is
* a possibility that an interrupt can make a 64 bit read non-atomic even
* when 8 byte aligned. So use fully memory barrier InterlockedOr64(). */
#define atomic_read64(SRC, DST, ORDER) \
__pragma (warning(push)) \
__pragma (warning(disable:4047)) \
*(DST) = InterlockedOr64((int64_t volatile *) (SRC), 0); \
__pragma (warning(pop))
On Tue, Mar 29, 2016 at 04:20:13PM +0000, Sorin Vinturis wrote:
> Ben, ovs-atomic-msvc.h contains support for 8,16,32 and 64-bit arithmetic and
> logical operations.
> Regarding 64-bit operations, like read and write, they are performed using
> specific 64-bit interlocked functions, i.e. InterlockedCompareExchange64 and,
> respectivelly, InterlockedOr64. Both functions are performed atomically.
>
> All remaining 64-bit operations use, also, 64-bit interlocked functions,
> which are executed atomically, except for logical AND operation. For the
> latter operation, an intrinsic function is used, _InterlockedExchangeAdd64,
> which means that is a faster function built in to the compiler. More details
> about this is here:
> https://msdn.microsoft.com/ro-ro/library/26td21ds(v=vs.80).aspx.
>
>
> -----Original Message-----
> From: Ben Pfaff [mailto:[email protected]]
> Sent: Tuesday, 29 March, 2016 00:25
> To: Sorin Vinturis
> Cc: Guru Shetty; [email protected]
> Subject: Re: [ovs-dev] [PATCH] datapath-windows: Add Windows thread atomic
> APIs for x64 binaries.
>
> On Mon, Mar 28, 2016 at 09:11:24PM +0000, Sorin Vinturis wrote:
> > Hi Guru,
> >
> > I have verified the ovs-atomic-msvc.h header and all the defined
> > macros and functions have 32-bit and 64-bit correspondent. All 64-bit
> > macros use InterlockedXXX functions that are atomic:
> > InterlockedExchangeNoFence64, InterlockedExchange64,
> > InterlockedCompareExchange64, _InterlockedExchangeAdd64,
> > InterlockedAnd64, InterlockedOr64, InterlockedXor64. I have ran
> > test-atomic.c unit tests, on both 32-bit and 64-bit platforms, and all
> > the tests are verified.
> >
> > I also tested the 64-bit build on Hyper-V and it is stable.
> >
> > Is there a reason why we should not use ovs-atomic-msvc APIs for 64-bit
> > builds?
>
> ovs-atomic-msvc.h seems pretty oriented toward a 32-bit build. At least, I
> really doubt MSVC would break 64-bit reads and writes into two 32-bit reads
> and writes on 64-bit builds, which is what the code in the header currently
> assumes. You don't want to update it to handle 64-bit builds differently? I
> guess that these would be optimizations, but they seem straightforward.
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