[lttng-dev] [PATCH 6/7] Fix: uatomic_or() need retyping to uintptr_t in rculfhash.c

[Mathieu Desnoyers]

On 2023-03-21 06:15, Ondřej Surý wrote:
> 
>> On 20. 3. 2023, at 19:31, Mathieu Desnoyers <mathieu.desnoyers at efficios.com> wrote:
>>
>> On 2023-03-17 17:37, Ondřej Surý via lttng-dev wrote:
>>> When adding REMOVED_FLAG to the pointers in the rculfhash
>>> implementation, retype the generic pointer to uintptr_t to fix the
>>> compiler error.
>>
>> What is the compiler error ? I'm wondering whether the expected choice
>> to match the rest of this file's content would be to use "uintptr_t *" or "unsigned long *" ?
> 
> This is the error:
> 
> rculfhash.c:1201:2: error: address argument to atomic operation must be a pointer to integer ('struct cds_lfht_node **' invalid)
>          uatomic_or(&node->next, REMOVED_FLAG);
>          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ../include/urcu/uatomic.h:60:8: note: expanded from macro 'uatomic_or'
>          (void)__atomic_or_fetch((addr), (mask), __ATOMIC_RELAXED)
>                ^                 ~~~~~~
> rculfhash.c:1444:3: error: address argument to atomic operation must be a pointer to integer ('struct cds_lfht_node **' invalid)
>                  uatomic_or(&fini_bucket->next, REMOVED_FLAG);
>                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ../include/urcu/uatomic.h:60:8: note: expanded from macro 'uatomic_or'
>          (void)__atomic_or_fetch((addr), (mask), __ATOMIC_RELAXED)
>                ^                 ~~~~~~
> 
> uintptr_t is defined as "unsigned integer type capable of holding a pointer to void" while unsigned long is at least 32-bit;
> 
> I guess that works in a practise, but using unsigned long to retype the pointers might blow up (thinking of x32 which I know
> little about, but it's kind of hybrid architecture, isn't it?)

x32 uses 4 bytes for unsigned long, uintptr_t, and void * size. So even 
that architecture is OK with casting pointer to unsigned long.

I agree with you that uintptr_t is the semantically correct type, but it 
should come as a separate change across the urcu code base: currently 
there are many places where void * is cast to unsigned long to do 
bitwise operations.

I therefore recommend to use unsigned long here to stay similar to the 
rest of the code base, and keep the transition from unsigned long to 
uintptr_t for the future, as it is not an immediate issue we have to 
address.

Thanks,

Mathieu


> 
> Ondrej
> --
> Ondřej Surý (He/Him)
> ondrej at sury.org
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com