Re: semtimedop, powerpc, time64 and older kernels

[Matheus Castanho <msc@linux.ibm.com>]

On 9/30/20 4:12 PM, Adhemerval Zanella wrote:
> 
> 
> On 30/09/2020 15:29, Matheus Castanho wrote:
>> Also, looks like my email client messed up the diff *sigh*. I'm sending
>> a proper patch attached this time.
>>
>> --
>> Matheus Castanho
>>
> 
>> From 1c0a497a3f986bc6980581c9eab482ccf7bb190f Mon Sep 17 00:00:00 2001
>> From: Matheus Castanho <msc@linux.ibm.com>
>> Date: Wed, 30 Sep 2020 14:22:18 -0300
>> Subject: [PATCH] sysvipc: Fix semtimedop for Linux < 5.1
>>
>> Kernels older than 5.1 will fail with ENOSYS when calling
>> semtimedop_time64 syscall in __semtimedop_time64. Just like for
>> !__ASSUME_TIME64_SYSCALLS, we should fallback to using the old mechanism
>> in such cases.
>> ---
>>  sysdeps/unix/sysv/linux/semtimedop.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/sysdeps/unix/sysv/linux/semtimedop.c b/sysdeps/unix/sysv/linux/semtimedop.c
>> index a9ad922ee2..510fea1852 100644
>> --- a/sysdeps/unix/sysv/linux/semtimedop.c
>> +++ b/sysdeps/unix/sysv/linux/semtimedop.c
>> @@ -32,7 +32,7 @@ __semtimedop64 (int semid, struct sembuf *sops, size_t nsops,
>>    int r = INLINE_SYSCALL_CALL (semtimedop_time64, semid, sops, nsops,
>>  			       timeout);
>>  
>> -#ifndef __ASSUME_TIME64_SYSCALLS
>> +#if !(defined __ASSUME_TIME64_SYSCALLS) || __LINUX_KERNEL_VERSION < 0x050100
>>    if (r == 0 || errno != ENOSYS)
>>      return r;
>>  
>> -- 
>> 2.26.2
> 
> Thanks for catching it and although it fixes the regression, we have 
> kernel-features.h exactly to avoid using __LINUX_KERNEL_VERSION through the
> implementations. Also this is sub-optimal since it forces semtimeopd issues
> __NR_semtimeop and then __NR_ipc on powerpc64 and we have 
> __ASSUME_DIRECT_SYSVIPC_SYSCALLS exactly to avoid this strategy of handling 
> ENOSYS for newer syscalls and thus slowing it down the implementation on 
> older kernels (--enable-kernel exists exactly to get rid of this older kernel
> support).
> 

Thanks for the explanation.

> I forgot that powerpc64 and s390x used the older multiplexed __NR_ipc and
> kernel v5.1 decided to add proper __NR_semtimedop (and it was in fact handled
> by 720e9541f5d919).  I think a better fix is the one below, since it:
> 
>   1. Issues __NR_semtimeop_time64 iff it is defined (32-bit architectures).
>   2. Issues __NR_semtimeop otherwise iff glibc is configured for a kernel that
>      supports it (for powerpc64 it will only for --enable-kernel=5.1). 
>      Otherwise it will use only 3.
>   3. Issues __NR_ipc with IPCOP_semtimedop.
> 
> For powerpc64 it will issue either __NR_ipc (default) or __NR_semtimeop
> (--enable-kernel=5.1), while for powerpc it will use either
> __NR_semtimeop_time64 and fallback to __NR_ipc or just issue
> __NR_semtimeop_time64.

Sounds good to me. That is indeed a better solution. Thanks!

> 
> I am running some regressions before commit it.
> 
> ---
> 
> diff --git a/sysdeps/unix/sysv/linux/semtimedop.c b/sysdeps/unix/sysv/linux/semtimedop.c
> index a9ad922ee2..29647f8ccd 100644
> --- a/sysdeps/unix/sysv/linux/semtimedop.c
> +++ b/sysdeps/unix/sysv/linux/semtimedop.c
> @@ -26,11 +26,15 @@ int
>  __semtimedop64 (int semid, struct sembuf *sops, size_t nsops,
>                 const struct __timespec64 *timeout)
>  {
> -#ifndef __NR_semtimedop_time64
> -# define __NR_semtimedop_time64 __NR_semtimedop
> +  int r;
> +#if defined __NR_semtimedop_time64
> +  r = INLINE_SYSCALL_CALL (semtimedop_time64, semid, sops, nsops, timeout);
> +#elif defined __ASSUME_DIRECT_SYSVIPC_SYSCALLS && defined __NR_semtimedop
> +  r = INLINE_SYSCALL_CALL (semtimedop, semid, sops, nsops, timeout);
> +#else
> +  r = INLINE_SYSCALL_CALL (ipc, IPCOP_semtimedop, semid,
> +                          SEMTIMEDOP_IPC_ARGS (nsops, sops, timeout));
>  #endif
> -  int r = INLINE_SYSCALL_CALL (semtimedop_time64, semid, sops, nsops,
> -                              timeout);
>  
>  #ifndef __ASSUME_TIME64_SYSCALLS
>    if (r == 0 || errno != ENOSYS)
> 

The change looks good and fixes the issue. Tested on ppc64le.

Reviewed-by: Matheus Castanho <msc@linux.ibm.com>

--
Matheus Castanho