On 16 ÍÁÊ, 21:41, "Dmitriy V'jukov" <dvyu...@gmail. comwrote:
Can you clarify about atomic_global_f ence_compatibil ity.
1.10/7 (N2521)
"An evaluation A that performs a release operation on an object M
synchronizes with an evaluation B that performs an acquire operation
on M and reads a value written by any side effect in the release
sequence headed by A."
Release operation is performed on 'obj->rc' and acquire operation is
performed on 'std::atomic_gl obal_fence_comp atibility'. So why the
former 'synchronizes with' the latter?
According to definition, release operation on
'std::atomic_gl obal_fence_comp atibility' can only 'synchronize with'
acquire operation on 'std::atomic_gl obal_fence_comp atibility'.
Imho, it's a bit senseless. I think that I miss something. Can
somebody clarify about std::atomic_glo bal_fence_compa tibility?
Dmitriy V'jukov 10  2344 
On May 17, 4:07 pm, "Dmitriy V'jukov" <dvyu...@gmail. comwrote:
On 16 ÍÁÊ, 21:41, "Dmitriy V'jukov" <dvyu...@gmail. comwrote:
Can you clarify about atomic_global_f ence_compatibil ity.
1.10/7 (N2521)
"An evaluation A that performs a release operation on an object M
synchronizes with an evaluation B that performs an acquire operation
on M and reads a value written by any side effect in the release
sequence headed by A."
Release operation is performed on 'obj->rc' and acquire operation is
performed on 'std::atomic_gl obal_fence_comp atibility'. So why the
former 'synchronizes with' the latter?
According to definition, release operation on
'std::atomic_gl obal_fence_comp atibility' can only 'synchronize with'
acquire operation on 'std::atomic_gl obal_fence_comp atibility'.
Imho, it's a bit senseless. I think that I miss something. Can
somebody clarify about std::atomic_glo bal_fence_compa tibility?
It's a global fence (see 29.3p11 in N2588). If that's not clear, then
that's a defect.
Anthony
"Dmitriy V'jukov" <dv*****@gmail. comwrites:
My current understanding is that:
1. atomic_global_f ence_compatibil ity must have some special treatment
from language.
or
2. it's useless.
My understanding is #1: it has special treatment. Use of this variable
for a fence is equivalent to a global fence instruction.
Anthony
--
Anthony Williams | Just Software Solutions Ltd
Custom Software Development | http://www.justsoftwaresolutions.co.uk
Registered in England, Company Number 5478976.
Registered Office: 15 Carrallack Mews, St Just, Cornwall, TR19 7UL
On May 20, 6:15 pm, "Dmitriy V'jukov" <dvyu...@gmail. comwrote:
2. Acquire operation must read value in 'release-sequence' of release
operation. What value "reads" fence() function? Since fence() is RMW
atomic operation, it must "read" some value. But how can I find out
what value it reads?
Btw, this applies to atomic_fence() function in general - what value
it "reads"?
If I don't know what value it reads then I can't be sure that
something 'synchronized-with'.
Dmitriy V'jukov
On May 20, 6:20 pm, anthony....@gma il.com wrote:
"Dmitriy V'jukov" <dvyu...@gmail. comwrites:
My current understanding is that:
1. atomic_global_f ence_compatibil ity must have some special treatment
from language.
or
2. it's useless.
My understanding is #1: it has special treatment. Use of this variable
for a fence is equivalent to a global fence instruction.
Where this 'special treatment' formally described?
29.4/11 (N2588) says only "... may be used to emulate global fences".
And 29.4/11 a *Note*.
I don't see any 'special treatment'. And according to 1.10 your
example is broken...
Dmitriy V'jukov
"Dmitriy V'jukov" <dv*****@gmail. comwrites:
On May 20, 6:15 pm, "Dmitriy V'jukov" <dvyu...@gmail. comwrote:
>2. Acquire operation must read value in 'release-sequence' of release
operation. What value "reads" fence() function? Since fence() is RMW
atomic operation, it must "read" some value. But how can I find out
what value it reads?
Btw, this applies to atomic_fence() function in general - what value
it "reads"?
You apply a fence on a specific variable, and it "reads" one of the
values written to the variable by some thread. A fence is a RMW
operation that writes the same value back.
If I don't know what value it reads then I can't be sure that
something 'synchronized-with'.
True. Fences need to be used carefully, and in combination with other
operations.
Anthony
--
Anthony Williams | Just Software Solutions Ltd
Custom Software Development | http://www.justsoftwaresolutions.co.uk
Registered in England, Company Number 5478976.
Registered Office: 15 Carrallack Mews, St Just, Cornwall, TR19 7UL
"Dmitriy V'jukov" <dv*****@gmail. comwrites:
On May 20, 6:20 pm, anthony....@gma il.com wrote:
>"Dmitriy V'jukov" <dvyu...@gmail. comwrites:
My current understanding is that:
1. atomic_global_f ence_compatibil ity must have some special treatment
from language.
or
2. it's useless.
My understanding is #1: it has special treatment. Use of this variable
for a fence is equivalent to a global fence instruction.
Where this 'special treatment' formally described?
29.4/11 (N2588) says only "... may be used to emulate global fences".
And 29.4/11 a *Note*.
I don't see any 'special treatment'. And according to 1.10 your
example is broken...
I've raised the issue on the LWG reflector. I'll let you know what
others say.
Anthony
--
Anthony Williams | Just Software Solutions Ltd
Custom Software Development | http://www.justsoftwaresolutions.co.uk
Registered in England, Company Number 5478976.
Registered Office: 15 Carrallack Mews, St Just, Cornwall, TR19 7UL
On May 20, 7:12 pm, anthony....@gma il.com wrote:
"Dmitriy V'jukov" <dvyu...@gmail. comwrites:
On May 20, 6:15 pm, "Dmitriy V'jukov" <dvyu...@gmail. comwrote:
2. Acquire operation must read value in 'release-sequence' of release
operation. What value "reads" fence() function? Since fence() is RMW
atomic operation, it must "read" some value. But how can I find out
what value it reads?
Btw, this applies to atomic_fence() function in general - what value
it "reads"?
You apply a fence on a specific variable, and it "reads" one of the
values written to the variable by some thread. A fence is a RMW
operation that writes the same value back.
If I don't know what value it reads then I can't be sure that
something 'synchronized-with'.
True. Fences need to be used carefully, and in combination with other
operations.
Ok. In following example I know what value fence operation will
"read". And luckily 'release sequence' is still not broken when
acquire operation executes.
void release(T* obj)
{
int rc = obj->rc.fetch_add (-1, memory_order_re lease) - 1;
if (0 == rc)
{
if (0 == obj->rc.fence(memor y_order_acquire ))
delete obj;
else
assert(false);
}
}
But let's consider Paul McKenney's motivating example from N2153
( http://www.open-std.org/jtc1/sc22/wg...07/n2153.pdf):
for (i=0; i< num_mailboxes; i++)
{
if (mailbox[i].load_raw() == my_id)
{
acquire_fence() ; // Prevents speculation of memory do_work(i);
// accesses in do_work
}
}
The goal is to remove acquire fence from every load. And issue acquire
fence only when there is real work to do.
As I understand following code won't work:
for (i=0; i< num_mailboxes; i++)
{
if (mailbox[i].load(std::memo ry_order_relaxe d) == my_id)
{
mailbox[i].fence(std::mem ory_order_acqui re); // Prevents
speculation of memory do_work(i);
// accesses in do_work
}
}
Because 'release sequence' is just broken by relaxed load. Right?
This code will work if definition of 'synchronizes with' will be
changed from:
"An evaluation A that performs a release operation on an object M
synchronizes with an evaluation B that performs an acquire operation
on M and reads a value written by any side effect in the release
sequence headed by A."
to:
"An evaluation A that performs a release operation on an object M
synchronizes with an evaluation B that performs an acquire operation
on M and B *or any expression sequenced before B* reads a value
written by any side effect in the release sequence headed by A."
What do you think?
Dmitriy V'jukov
"Dmitriy V'jukov" <dv*****@gmail. comwrites:
Ok. In following example I know what value fence operation will
"read". And luckily 'release sequence' is still not broken when
acquire operation executes.
void release(T* obj)
{
int rc = obj->rc.fetch_add (-1, memory_order_re lease) - 1;
if (0 == rc)
{
if (0 == obj->rc.fence(memor y_order_acquire ))
delete obj;
else
assert(false);
}
}
I'm happy that this example is fine.
But let's consider Paul McKenney's motivating example from N2153
(http://www.open-std.org/jtc1/sc22/wg...07/n2153.pdf):
for (i=0; i< num_mailboxes; i++)
{
if (mailbox[i].load_raw() == my_id)
{
acquire_fence() ; // Prevents speculation of memory do_work(i);
// accesses in do_work
}
}
The goal is to remove acquire fence from every load. And issue acquire
fence only when there is real work to do.
As I understand following code won't work:
for (i=0; i< num_mailboxes; i++)
{
if (mailbox[i].load(std::memo ry_order_relaxe d) == my_id)
{
mailbox[i].fence(std::mem ory_order_acqui re); // Prevents
speculation of memory do_work(i);
// accesses in do_work
}
}
Because 'release sequence' is just broken by relaxed load. Right?
The relaxed load is just that: a load. It therefore does not feature
in the "modificati on sequence" of mailbox[i], and does not break the
release sequence. Therefore the store/release that writes mailbox[i]
can still synchronize-with the fence.
I think this example does in fact work.
This code will work if definition of 'synchronizes with' will be
changed from:
"An evaluation A that performs a release operation on an object M
synchronizes with an evaluation B that performs an acquire operation
on M and reads a value written by any side effect in the release
sequence headed by A."
to:
"An evaluation A that performs a release operation on an object M
synchronizes with an evaluation B that performs an acquire operation
on M and B *or any expression sequenced before B* reads a value
written by any side effect in the release sequence headed by A."
What do you think?
I think that is unnecessary. A relaxed load doesn't interfere with the
release sequence or the modification sequence, and if the relaxed load
sees the value, a subsequent acquire on the same thread can only see
that value or a later value (1.10p10 in N2588).
Anthony
--
Anthony Williams | Just Software Solutions Ltd
Custom Software Development | http://www.justsoftwaresolutions.co.uk
Registered in England, Company Number 5478976.
Registered Office: 15 Carrallack Mews, St Just, Cornwall, TR19 7UL
On May 20, 8:35 pm, anthony....@gma il.com wrote:
Because 'release sequence' is just broken by relaxed load. Right?
The relaxed load is just that: a load. It therefore does not feature
in the "modificati on sequence" of mailbox[i], and does not break the
release sequence. Therefore the store/release that writes mailbox[i]
can still synchronize-with the fence.
Yikes! You are right!
I think this example does in fact work.
Ok. Things start clearing up :)
This code will work if definition of 'synchronizes with' will be
changed from:
"An evaluation A that performs a release operation on an object M
synchronizes with an evaluation B that performs an acquire operation
on M and reads a value written by any side effect in the release
sequence headed by A."
to:
"An evaluation A that performs a release operation on an object M
synchronizes with an evaluation B that performs an acquire operation
on M and B *or any expression sequenced before B* reads a value
written by any side effect in the release sequence headed by A."
What do you think?
I think that is unnecessary. A relaxed load doesn't interfere with the
release sequence or the modification sequence, and if the relaxed load
sees the value, a subsequent acquire on the same thread can only see
that value or a later value (1.10p10 in N2588).
Yes, it already works figuratively this way:
1. Load function 'reads the value'
2. Subsequent acquire fence 'performs an acquire operation'
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