From: Julien Grall <jgrall@amazon.com>

Even if we assigned pirq->arch.irq to a variable, a compile is still
allowed to read pirq->arch.irq multiple time. This means that the value
checked may be different from the value used to get the desc.

Force the compiler to only do one read access by using read_atomic().

Signed-off-by: Julien Grall <jgrall@amazon.com>
---
 xen/arch/x86/irq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
index a69937c840b9..25f2eb611692 100644
--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
 
     for ( ; ; )
     {
-        int irq = pirq->arch.irq;
+        int irq = read_atomic(&pirq->arch.irq);
 
         if ( irq <= 0 )
             return NULL;
-- 
2.17.1

On 22.07.2020 18:53, Julien Grall wrote:
> --- a/xen/arch/x86/irq.c
> +++ b/xen/arch/x86/irq.c
> @@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
>  
>      for ( ; ; )
>      {
> -        int irq = pirq->arch.irq;
> +        int irq = read_atomic(&pirq->arch.irq);

There we go - I'd be fine this way, but I'm pretty sure Andrew
would want this to be ACCESS_ONCE(). So I guess now is the time
to settle which one to prefer in new code (or which criteria
there are to prefer one over the other).

And this is of course besides the fact that I think we have many
more instances where guaranteeing a single access would be
needed, if we're afraid of the described permitted compiler
behavior. Which then makes me wonder if this is really something
we should fix one by one, rather than by at least larger scope
audits (in order to not suggest "throughout the code base").

As a minor remark, unless you've observed problematic behavior,
would you mind adding "potential" or "theoretical" to the title?

Jan

Hi Jan,

On 23/07/2020 12:23, Jan Beulich wrote:
> On 22.07.2020 18:53, Julien Grall wrote:
>> --- a/xen/arch/x86/irq.c
>> +++ b/xen/arch/x86/irq.c
>> @@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
>>   
>>       for ( ; ; )
>>       {
>> -        int irq = pirq->arch.irq;
>> +        int irq = read_atomic(&pirq->arch.irq);
> 
> There we go - I'd be fine this way, but I'm pretty sure Andrew
> would want this to be ACCESS_ONCE(). So I guess now is the time
> to settle which one to prefer in new code (or which criteria
> there are to prefer one over the other).

I would prefer if we have a single way to force the compiler to do a 
single access (read/write).

The existing implementation of ACCESS_ONCE() can only work on scalar 
type. The implementation is based on a Linux, although we have an extra 
check. Looking through the Linux history, it looks like it is not 
possible to make ACCESS_ONCE() work with non-scalar types:

     ACCESS_ONCE does not work reliably on non-scalar types. For
     example gcc 4.6 and 4.7 might remove the volatile tag for such
     accesses during the SRA (scalar replacement of aggregates) step
     https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145)

I understand that our implementation of read_atomic(), write_atomic() 
would lead to less optimized code. So maybe we want to import 
READ_ONCE() and WRITE_ONCE() from Linux?

As a side note, I have seen suggestion only (see [1]) which suggest that 
they those helpers wouldn't be portable:

"One relatively unimportant misunderstanding is due to the fact that the 
standard only talks about accesses to volatile objects. It does not talk 
about accesses via volatile qualified pointers. Some programmers believe 
that using a pointer-to-volatile should be handled as though it pointed 
to a volatile object. That is not guaranteed by the standard and is 
therefore not portable. However, this is relatively unimportant because 
gcc does in fact treat a pointer-to-volatile as though it pointed to a 
volatile object."

I would assume that the use is OK on CLang and GCC given that Linux has 
been using it.


> And this is of course besides the fact that I think we have many
> more instances where guaranteeing a single access would be
> needed, if we're afraid of the described permitted compiler
> behavior. Which then makes me wonder if this is really something
> we should fix one by one, rather than by at least larger scope
> audits (in order to not suggest "throughout the code base").

It depends how much the contributor can invest on chasing the rest of 
the issues. The larger the scope is, the less likely you will find 
someone that has bandwith to allocate for fixing it completely.

If the scope is "a field", then I think it is a reasonable suggesting.

In this case, I had a look at arch.irq and wasn't able to spot other 
potential issue.

> As a minor remark, unless you've observed problematic behavior,
> would you mind adding "potential" or "theoretical" to the title?

I am not aware of any issues with compiler so far. So I can add 
"potential" in the title.

Cheers,

[1] https://www.airs.com/blog/archives/154

-- 
Julien Grall

On 23.07.2020 15:22, Julien Grall wrote:
> On 23/07/2020 12:23, Jan Beulich wrote:
>> On 22.07.2020 18:53, Julien Grall wrote:
>>> --- a/xen/arch/x86/irq.c
>>> +++ b/xen/arch/x86/irq.c
>>> @@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
>>>   
>>>       for ( ; ; )
>>>       {
>>> -        int irq = pirq->arch.irq;
>>> +        int irq = read_atomic(&pirq->arch.irq);
>>
>> There we go - I'd be fine this way, but I'm pretty sure Andrew
>> would want this to be ACCESS_ONCE(). So I guess now is the time
>> to settle which one to prefer in new code (or which criteria
>> there are to prefer one over the other).
> 
> I would prefer if we have a single way to force the compiler to do a 
> single access (read/write).

Ideally yes. I'm unconvinced though that either construct fits all
needs (for {read,write}_atomic() there may be reasons why the
compiler is allowed to produce multiple generated code instances
from a single source instance, while for *_ONCE() the compiler may
be allowed to split the access into pieces (as can be easily seen
for an access to a uint64_t variable on 32-bit x86 at least, and
by deduction I then can't see why it shouldn't be allowed to use
byte-wise accesses).

> The existing implementation of ACCESS_ONCE() can only work on scalar 
> type. The implementation is based on a Linux, although we have an extra 
> check. Looking through the Linux history, it looks like it is not 
> possible to make ACCESS_ONCE() work with non-scalar types:
> 
>      ACCESS_ONCE does not work reliably on non-scalar types. For
>      example gcc 4.6 and 4.7 might remove the volatile tag for such
>      accesses during the SRA (scalar replacement of aggregates) step
>      https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145)
> 
> I understand that our implementation of read_atomic(), write_atomic() 
> would lead to less optimized code.

I.e. you see ways for the compiler to be more clever than using
a single "move" instruction for a single move? Or are you referring
to insn scheduling by the compiler (which my gut feeling would say
is impacted as much by an asm volatile() as by accessing a volatile
object).

> So maybe we want to import 
> READ_ONCE() and WRITE_ONCE() from Linux?

So far I was under the impression that our ACCESS_ONCE() is the
result of folding (older) Linux'es READ_ONCE() and WRITE_ONCE()
into a single construct.

> As a side note, I have seen suggestion only (see [1]) which suggest that 
> they those helpers wouldn't be portable:
> 
> "One relatively unimportant misunderstanding is due to the fact that the 
> standard only talks about accesses to volatile objects. It does not talk 
> about accesses via volatile qualified pointers. Some programmers believe 
> that using a pointer-to-volatile should be handled as though it pointed 
> to a volatile object. That is not guaranteed by the standard and is 
> therefore not portable. However, this is relatively unimportant because 
> gcc does in fact treat a pointer-to-volatile as though it pointed to a 
> volatile object."
> 
> I would assume that the use is OK on CLang and GCC given that Linux has 
> been using it.

Then again your change here is exactly to drop such assumptions of
ours on compiler behavior.

>> And this is of course besides the fact that I think we have many
>> more instances where guaranteeing a single access would be
>> needed, if we're afraid of the described permitted compiler
>> behavior. Which then makes me wonder if this is really something
>> we should fix one by one, rather than by at least larger scope
>> audits (in order to not suggest "throughout the code base").
> 
> It depends how much the contributor can invest on chasing the rest of 
> the issues. The larger the scope is, the less likely you will find 
> someone that has bandwith to allocate for fixing it completely.

I certainly understand that.

> If the scope is "a field", then I think it is a reasonable suggesting.
> 
> In this case, I had a look at arch.irq and wasn't able to spot other 
> potential issue.

That's good to know, and may be worth mentioning - if not in the
description, then maybe in a post-commit-message remark?

Jan

On 23/07/2020 14:22, Julien Grall wrote:
> Hi Jan,
>
> On 23/07/2020 12:23, Jan Beulich wrote:
>> On 22.07.2020 18:53, Julien Grall wrote:
>>> --- a/xen/arch/x86/irq.c
>>> +++ b/xen/arch/x86/irq.c
>>> @@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
>>>         for ( ; ; )
>>>       {
>>> -        int irq = pirq->arch.irq;
>>> +        int irq = read_atomic(&pirq->arch.irq);
>>
>> There we go - I'd be fine this way, but I'm pretty sure Andrew
>> would want this to be ACCESS_ONCE(). So I guess now is the time
>> to settle which one to prefer in new code (or which criteria
>> there are to prefer one over the other).
>
> I would prefer if we have a single way to force the compiler to do a
> single access (read/write).

Unlikely to happen, I'd expect.

But I would really like to get rid of (or at least rename)
read_atomic()/write_atomic() specifically because they've got nothing to
do with atomic_t's and the set of functionality who's namespace they share.

>
> The existing implementation of ACCESS_ONCE() can only work on scalar
> type. The implementation is based on a Linux, although we have an
> extra check. Looking through the Linux history, it looks like it is
> not possible to make ACCESS_ONCE() work with non-scalar types:
>
>     ACCESS_ONCE does not work reliably on non-scalar types. For
>     example gcc 4.6 and 4.7 might remove the volatile tag for such
>     accesses during the SRA (scalar replacement of aggregates) step
>     https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145)
>
> I understand that our implementation of read_atomic(), write_atomic()
> would lead to less optimized code.

There are cases where read_atomic()/write_atomic() prevent optimisations
which ACCESS_ONCE() would allow, but it is only for code of the form:

ACCESS_ONCE(ptr) |= val;

Which a sufficiently clever compiler could convert to a single `or $val,
ptr` instruction on x86, while read_atomic()/write_atomic() would force
it to be `mov ptr, %reg; or $val, %reg; mov %reg, ptr`.

That said - your note about GCC treating the pointed-to object as
volatile probably means it won't make the above optimisation, even
though it would be appropriate to do so.

> So maybe we want to import READ_ONCE() and WRITE_ONCE() from Linux?

There is no point.  Linux has taken a massive detour through wildly
different READ/WRITE_ONCE() functions (to fix the above GCC bugs), and
are now back to something very similar to ACCESS_ONCE().

~Andrew

Hi Andrew,

Sorry for the late answer.

On 23/07/2020 14:59, Andrew Cooper wrote:
> On 23/07/2020 14:22, Julien Grall wrote:
>> Hi Jan,
>>
>> On 23/07/2020 12:23, Jan Beulich wrote:
>>> On 22.07.2020 18:53, Julien Grall wrote:
>>>> --- a/xen/arch/x86/irq.c
>>>> +++ b/xen/arch/x86/irq.c
>>>> @@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
>>>>          for ( ; ; )
>>>>        {
>>>> -        int irq = pirq->arch.irq;
>>>> +        int irq = read_atomic(&pirq->arch.irq);
>>>
>>> There we go - I'd be fine this way, but I'm pretty sure Andrew
>>> would want this to be ACCESS_ONCE(). So I guess now is the time
>>> to settle which one to prefer in new code (or which criteria
>>> there are to prefer one over the other).
>>
>> I would prefer if we have a single way to force the compiler to do a
>> single access (read/write).
> 
> Unlikely to happen, I'd expect.
> 
> But I would really like to get rid of (or at least rename)
> read_atomic()/write_atomic() specifically because they've got nothing to
> do with atomic_t's and the set of functionality who's namespace they share.

Would you be happy if I rename both to READ_ONCE() and WRITE_ONCE()? I 
would also suggest to move them implementation in a new header asm/lib.h.

Cheers,

-- 
Julien Grall

On 14.08.2020 21:25, Julien Grall wrote:
> Hi Andrew,
> 
> Sorry for the late answer.
> 
> On 23/07/2020 14:59, Andrew Cooper wrote:
>> On 23/07/2020 14:22, Julien Grall wrote:
>>> Hi Jan,
>>>
>>> On 23/07/2020 12:23, Jan Beulich wrote:
>>>> On 22.07.2020 18:53, Julien Grall wrote:
>>>>> --- a/xen/arch/x86/irq.c
>>>>> +++ b/xen/arch/x86/irq.c
>>>>> @@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
>>>>>          for ( ; ; )
>>>>>        {
>>>>> -        int irq = pirq->arch.irq;
>>>>> +        int irq = read_atomic(&pirq->arch.irq);
>>>>
>>>> There we go - I'd be fine this way, but I'm pretty sure Andrew
>>>> would want this to be ACCESS_ONCE(). So I guess now is the time
>>>> to settle which one to prefer in new code (or which criteria
>>>> there are to prefer one over the other).
>>>
>>> I would prefer if we have a single way to force the compiler to do a
>>> single access (read/write).
>>
>> Unlikely to happen, I'd expect.
>>
>> But I would really like to get rid of (or at least rename)
>> read_atomic()/write_atomic() specifically because they've got nothing to
>> do with atomic_t's and the set of functionality who's namespace they share.
> 
> Would you be happy if I rename both to READ_ONCE() and WRITE_ONCE()?

Wouldn't this lead to confusion with Linux'es macros of the same names?

> I would also suggest to move them implementation in a new header asm/lib.h.

Probably.

Jan

Hi Jan,

On 18/08/2020 09:39, Jan Beulich wrote:
> On 14.08.2020 21:25, Julien Grall wrote:
>> Hi Andrew,
>>
>> Sorry for the late answer.
>>
>> On 23/07/2020 14:59, Andrew Cooper wrote:
>>> On 23/07/2020 14:22, Julien Grall wrote:
>>>> Hi Jan,
>>>>
>>>> On 23/07/2020 12:23, Jan Beulich wrote:
>>>>> On 22.07.2020 18:53, Julien Grall wrote:
>>>>>> --- a/xen/arch/x86/irq.c
>>>>>> +++ b/xen/arch/x86/irq.c
>>>>>> @@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
>>>>>>           for ( ; ; )
>>>>>>         {
>>>>>> -        int irq = pirq->arch.irq;
>>>>>> +        int irq = read_atomic(&pirq->arch.irq);
>>>>>
>>>>> There we go - I'd be fine this way, but I'm pretty sure Andrew
>>>>> would want this to be ACCESS_ONCE(). So I guess now is the time
>>>>> to settle which one to prefer in new code (or which criteria
>>>>> there are to prefer one over the other).
>>>>
>>>> I would prefer if we have a single way to force the compiler to do a
>>>> single access (read/write).
>>>
>>> Unlikely to happen, I'd expect.
>>>
>>> But I would really like to get rid of (or at least rename)
>>> read_atomic()/write_atomic() specifically because they've got nothing to
>>> do with atomic_t's and the set of functionality who's namespace they share.
>>
>> Would you be happy if I rename both to READ_ONCE() and WRITE_ONCE()?
> 
> Wouldn't this lead to confusion with Linux'es macros of the same names?

 From my understanding, the purpose of READ_ONCE()/WRITE_ONCE() in Linux 
is the same as our read_atomic()/write_atomic().

So I think it would be fine to rename them. An alternative would be port 
the Linux version in Xen and drop ours.

Cheers,

-- 
Julien Grall

On 18.08.2020 10:53, Julien Grall wrote:
> Hi Jan,
> 
> On 18/08/2020 09:39, Jan Beulich wrote:
>> On 14.08.2020 21:25, Julien Grall wrote:
>>> Hi Andrew,
>>>
>>> Sorry for the late answer.
>>>
>>> On 23/07/2020 14:59, Andrew Cooper wrote:
>>>> On 23/07/2020 14:22, Julien Grall wrote:
>>>>> Hi Jan,
>>>>>
>>>>> On 23/07/2020 12:23, Jan Beulich wrote:
>>>>>> On 22.07.2020 18:53, Julien Grall wrote:
>>>>>>> --- a/xen/arch/x86/irq.c
>>>>>>> +++ b/xen/arch/x86/irq.c
>>>>>>> @@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
>>>>>>>           for ( ; ; )
>>>>>>>         {
>>>>>>> -        int irq = pirq->arch.irq;
>>>>>>> +        int irq = read_atomic(&pirq->arch.irq);
>>>>>>
>>>>>> There we go - I'd be fine this way, but I'm pretty sure Andrew
>>>>>> would want this to be ACCESS_ONCE(). So I guess now is the time
>>>>>> to settle which one to prefer in new code (or which criteria
>>>>>> there are to prefer one over the other).
>>>>>
>>>>> I would prefer if we have a single way to force the compiler to do a
>>>>> single access (read/write).
>>>>
>>>> Unlikely to happen, I'd expect.
>>>>
>>>> But I would really like to get rid of (or at least rename)
>>>> read_atomic()/write_atomic() specifically because they've got nothing to
>>>> do with atomic_t's and the set of functionality who's namespace they share.
>>>
>>> Would you be happy if I rename both to READ_ONCE() and WRITE_ONCE()?
>>
>> Wouldn't this lead to confusion with Linux'es macros of the same names?
> 
> From my understanding, the purpose of READ_ONCE()/WRITE_ONCE() in Linux is the same as our read_atomic()/write_atomic().
> 
> So I think it would be fine to rename them. An alternative would be port the Linux version in Xen and drop ours.

The port of Linux'es {READ,WRITE}_ONCE() is our ACCESS_ONCE(). As pointed
out before, ACCESS_ONCE() and {read,write}_atomic() serve slightly
different purposes, and so far it looks like all of us are lacking ideas
on how to construct something that catches all cases by one single approach.

Jan

Hi Jan,

On 18/08/2020 09:57, Jan Beulich wrote:
> On 18.08.2020 10:53, Julien Grall wrote:
>> Hi Jan,
>>
>> On 18/08/2020 09:39, Jan Beulich wrote:
>>> On 14.08.2020 21:25, Julien Grall wrote:
>>>> Hi Andrew,
>>>>
>>>> Sorry for the late answer.
>>>>
>>>> On 23/07/2020 14:59, Andrew Cooper wrote:
>>>>> On 23/07/2020 14:22, Julien Grall wrote:
>>>>>> Hi Jan,
>>>>>>
>>>>>> On 23/07/2020 12:23, Jan Beulich wrote:
>>>>>>> On 22.07.2020 18:53, Julien Grall wrote:
>>>>>>>> --- a/xen/arch/x86/irq.c
>>>>>>>> +++ b/xen/arch/x86/irq.c
>>>>>>>> @@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
>>>>>>>>            for ( ; ; )
>>>>>>>>          {
>>>>>>>> -        int irq = pirq->arch.irq;
>>>>>>>> +        int irq = read_atomic(&pirq->arch.irq);
>>>>>>>
>>>>>>> There we go - I'd be fine this way, but I'm pretty sure Andrew
>>>>>>> would want this to be ACCESS_ONCE(). So I guess now is the time
>>>>>>> to settle which one to prefer in new code (or which criteria
>>>>>>> there are to prefer one over the other).
>>>>>>
>>>>>> I would prefer if we have a single way to force the compiler to do a
>>>>>> single access (read/write).
>>>>>
>>>>> Unlikely to happen, I'd expect.
>>>>>
>>>>> But I would really like to get rid of (or at least rename)
>>>>> read_atomic()/write_atomic() specifically because they've got nothing to
>>>>> do with atomic_t's and the set of functionality who's namespace they share.
>>>>
>>>> Would you be happy if I rename both to READ_ONCE() and WRITE_ONCE()?
>>>
>>> Wouldn't this lead to confusion with Linux'es macros of the same names?
>>
>>  From my understanding, the purpose of READ_ONCE()/WRITE_ONCE() in Linux is the same as our read_atomic()/write_atomic().
>>
>> So I think it would be fine to rename them. An alternative would be port the Linux version in Xen and drop ours.
> 
> The port of Linux'es {READ,WRITE}_ONCE() is our ACCESS_ONCE().

Not really... Our ACCESS_ONCE() only supports scalar type. {READ, 
WRITE}_ONCE() are able to support non-scalar type as well.

> As pointed
> out before, ACCESS_ONCE() and {read,write}_atomic() serve slightly
> different purposes, and so far it looks like all of us are lacking ideas
> on how to construct something that catches all cases by one single approach.

I am guessing you are referring to [1], right?

If you read the documentation of READ_ONCE()/WRITE_ONCE(), they are 
meant to be atomic in some cases. The cases are exactly the same as 
{read, write}_atomic().

I will ask the same thing as I asked to Roger. If Linux can rely on it, 
why can't we?

Although, I agree that the implementation is not portable to another 
compiler. But that's why they are implemented in compiler.h.

Cheers,

[1] <d3ba0dad-63db-06ad-ff3f-f90fe8649845@suse.com>

> 


> Jan
> 

-- 
Julien Grall

On 18.08.2020 11:12, Julien Grall wrote:
> Hi Jan,
> 
> On 18/08/2020 09:57, Jan Beulich wrote:
>> On 18.08.2020 10:53, Julien Grall wrote:
>>> Hi Jan,
>>>
>>> On 18/08/2020 09:39, Jan Beulich wrote:
>>>> On 14.08.2020 21:25, Julien Grall wrote:
>>>>> Hi Andrew,
>>>>>
>>>>> Sorry for the late answer.
>>>>>
>>>>> On 23/07/2020 14:59, Andrew Cooper wrote:
>>>>>> On 23/07/2020 14:22, Julien Grall wrote:
>>>>>>> Hi Jan,
>>>>>>>
>>>>>>> On 23/07/2020 12:23, Jan Beulich wrote:
>>>>>>>> On 22.07.2020 18:53, Julien Grall wrote:
>>>>>>>>> --- a/xen/arch/x86/irq.c
>>>>>>>>> +++ b/xen/arch/x86/irq.c
>>>>>>>>> @@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
>>>>>>>>>            for ( ; ; )
>>>>>>>>>          {
>>>>>>>>> -        int irq = pirq->arch.irq;
>>>>>>>>> +        int irq = read_atomic(&pirq->arch.irq);
>>>>>>>>
>>>>>>>> There we go - I'd be fine this way, but I'm pretty sure Andrew
>>>>>>>> would want this to be ACCESS_ONCE(). So I guess now is the time
>>>>>>>> to settle which one to prefer in new code (or which criteria
>>>>>>>> there are to prefer one over the other).
>>>>>>>
>>>>>>> I would prefer if we have a single way to force the compiler to do a
>>>>>>> single access (read/write).
>>>>>>
>>>>>> Unlikely to happen, I'd expect.
>>>>>>
>>>>>> But I would really like to get rid of (or at least rename)
>>>>>> read_atomic()/write_atomic() specifically because they've got nothing to
>>>>>> do with atomic_t's and the set of functionality who's namespace they share.
>>>>>
>>>>> Would you be happy if I rename both to READ_ONCE() and WRITE_ONCE()?
>>>>
>>>> Wouldn't this lead to confusion with Linux'es macros of the same names?
>>>
>>>  From my understanding, the purpose of READ_ONCE()/WRITE_ONCE() in Linux is the same as our read_atomic()/write_atomic().
>>>
>>> So I think it would be fine to rename them. An alternative would be port the Linux version in Xen and drop ours.
>>
>> The port of Linux'es {READ,WRITE}_ONCE() is our ACCESS_ONCE().
> 
> Not really... Our ACCESS_ONCE() only supports scalar type. {READ, WRITE}_ONCE() are able to support non-scalar type as well.

I guess that's merely because ours was derived from an earlier version of
Linux.

>> As pointed
>> out before, ACCESS_ONCE() and {read,write}_atomic() serve slightly
>> different purposes, and so far it looks like all of us are lacking ideas
>> on how to construct something that catches all cases by one single approach.
> 
> I am guessing you are referring to [1], right?
> 
> If you read the documentation of READ_ONCE()/WRITE_ONCE(), they are meant to be atomic in some cases. The cases are exactly the same as {read, write}_atomic().
> 
> I will ask the same thing as I asked to Roger. If Linux can rely on it, why can't we?

That's not the way I'd like to see arguments go here: If Linux has
something suitable, I'm fine with us using it. But we ought to be
permitted to question whether what we inherit is indeed fit for
purpose, and afaict there is at least one gap to be filled. In no
case should we blindly pull in things from Linux and then assume
that simply by doing so all is well.

> Although, I agree that the implementation is not portable to another compiler. But that's why they are implemented in compiler.h.

Aiui items in compiler.h are meant to be suitable for all compilers,
whereas truly gcc-specific things (for example) live in
compiler-gcc.h (where Linux has two further ones they support).

Jan

On 18/08/2020 12:27, Jan Beulich wrote:
> On 18.08.2020 11:12, Julien Grall wrote:
>>> As pointed
>>> out before, ACCESS_ONCE() and {read,write}_atomic() serve slightly
>>> different purposes, and so far it looks like all of us are lacking ideas
>>> on how to construct something that catches all cases by one single approach.
>>
>> I am guessing you are referring to [1], right?
>>
>> If you read the documentation of READ_ONCE()/WRITE_ONCE(), they are meant to be atomic in some cases. The cases are exactly the same as {read, write}_atomic().
>>
>> I will ask the same thing as I asked to Roger. If Linux can rely on it, why can't we?
> 
> That's not the way I'd like to see arguments go here: If Linux has
> something suitable, I'm fine with us using it. But we ought to be
> permitted to question whether what we inherit is indeed fit for
> purpose, and afaict there is at least one gap to be filled. In no
> case should we blindly pull in things from Linux and then assume
> that simply by doing so all is well.

I don't think any of us here are compilers guru, so I would tend to rely 
on Linux memory work. After all their code received much more attention. 
But sure we can question everything they have been doing.

To me the expected semantics (/!\ I am not referring to the 
implementation) for all the helpers are the same. But you seem to 
disagree on that.

I read the thread again and I couldn't find any explanation how a 
developper could chose between ACCESS_ONCE() and {read, write}_atomic().

Can you outline how one would decide?

Cheers,

-- 
Julien Grall

On 18.08.2020 13:52, Julien Grall wrote:
> 
> 
> On 18/08/2020 12:27, Jan Beulich wrote:
>> On 18.08.2020 11:12, Julien Grall wrote:
>>>> As pointed
>>>> out before, ACCESS_ONCE() and {read,write}_atomic() serve slightly
>>>> different purposes, and so far it looks like all of us are lacking ideas
>>>> on how to construct something that catches all cases by one single approach.
>>>
>>> I am guessing you are referring to [1], right?
>>>
>>> If you read the documentation of READ_ONCE()/WRITE_ONCE(), they are meant to be atomic in some cases. The cases are exactly the same as {read, write}_atomic().
>>>
>>> I will ask the same thing as I asked to Roger. If Linux can rely on it, why can't we?
>>
>> That's not the way I'd like to see arguments go here: If Linux has
>> something suitable, I'm fine with us using it. But we ought to be
>> permitted to question whether what we inherit is indeed fit for
>> purpose, and afaict there is at least one gap to be filled. In no
>> case should we blindly pull in things from Linux and then assume
>> that simply by doing so all is well.
> 
> I don't think any of us here are compilers guru, so I would tend to rely on Linux memory work. After all their code received much more attention. But sure we can question everything they have been doing.
> 
> To me the expected semantics (/!\ I am not referring to the implementation) for all the helpers are the same. But you seem to disagree on that.
> 
> I read the thread again and I couldn't find any explanation how a developper could chose between ACCESS_ONCE() and {read, write}_atomic().
> 
> Can you outline how one would decide?

As long as both have weaknesses I'm afraid it's really a case-by-case
decision. If you're strictly after only the property that one has and
the other doesn't, the case is clear. In all other cases it'll need
weighing of the risks. No clear rules, I'm afraid.

Jan