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Re: [PATCH 5/7] linux: Use waitid on wait4 if __NR_wait4 is not defined
On 21/11/2019 15:41, Alistair Francis wrote:
> On Thu, Nov 21, 2019 at 9:53 AM Adhemerval Zanella
> <adhemerval.zanella@linaro.org> wrote:
>>
>>
>>
>> On 14/11/2019 11:47, Adhemerval Zanella wrote:
>>> +pid_t
>>> +__wait4 (pid_t pid, int *stat_loc, int options, struct rusage *usage)
>>> +{
>>> +#if __NR_wait4
>>> + return SYSCALL_CANCEL (wait4, pid, stat_loc, options, usage);
>>> +#elif defined (__ASSUME_WAITID_PID0_P_PGID)
>> [...]
>>> +# else
>>> +/* Linux waitid prior kernel 5.4 does not support waiting for the current
>>> + process. It would be possible to emulate it by calling getpgid for pid 0,
>>> + however, it would require an additional syscall and it is inherent racy:
>>> + after the current process group is received and before it is passed
>>> + to waitid a signal could arrive causing the current process group to
>>> + change. */
>>> +# error "The kernel ABI does not provide a way to implement wait4"
>>> +#endif
>>
>> So the only design here that I am not sure is if the best one is to trigger
>> a build error to avoid an architecture to not define __NR_wait4 and also
>> support kernels older than 5.4 (which would not define
>> __ASSUME_WAITID_PID0_P_PGID), or if it should do as generic implementation
>> and return ENOSYS along with a stub.
>>
>> Thoughts?
>
> I think a build error makes sense. Currently only RV32 doesn't have
> __NR_wait4 (which isn't upstreamed) so you aren't breaking anything.
>
> The only kernels that could possibly not have __NR_wait4 and be less
> then 5.4 are 5.1, 5.2 and 5.3, non of which are stable so they will
> slowly disappear anyway.
>
> Not producing a build error could be very confusing for developers
> that do get bitten by the missing implementation.
>
My point if if checking for kernel version to define __ASSUME_WAITID_PID0_P_PGID
does make, meaning it is possible with some config option in the kernel
to enable only waitid for kernels older than 5.3; or if we can assume
some configuration in always invalid and thus the kernel won't allow
enable it.
If the latter we can then remove the __ASSUME_WAITID_PID0_P_PGID and
add a comment on waitid implementation stating that if waitid is the
only syscall supported then it is suppose to be the superset of all
wait* functionalities.