Re: [committed] hppa: undef __ASSUME_SET_ROBUST_LIST

[Florian Weimer <fweimer@redhat.com>]

* Adhemerval Zanella Netto via Libc-alpha:

> On 20/09/22 17:20, John David Anglin wrote:
>> QEMU does not support support set_robust_list. Thus, we need
>> to enable detection of set_robust_list system call.
>> 
>> Signed-off-by: John David Anglin <dave.anglin@bell.net>
>> ---
>> diff --git a/sysdeps/unix/sysv/linux/hppa/kernel-features.h b/sysdeps/unix/sysv/linux/hppa/kernel-features.h
>> index 0cd21ef0fa..079612e4aa 100644
>> --- a/sysdeps/unix/sysv/linux/hppa/kernel-features.h
>> +++ b/sysdeps/unix/sysv/linux/hppa/kernel-features.h
>> @@ -30,3 +30,6 @@
>>  
>>  #undef __ASSUME_CLONE_DEFAULT
>>  #define __ASSUME_CLONE_BACKWARDS 1
>> +
>> +/* QEMU does not support set_robust_list.  */
>> +#undef __ASSUME_SET_ROBUST_LIST
>> 
>
> Does it happen only for hppa? If is a generic missing feature maybe we should
> undef __ASSUME_SET_ROBUST_LIST for all architectures.

I think it's just a 32-bit/64-bit mismatch issue.  You can't create
32-bit robust futexes from a 64-bit process because the pointer size in
the robust list differs.  For non-process-shared mutexes, qemu-user
should be able to emulate these futexes in userspace.  It already has to
intercept thread exit, and it can walk the mutex list at that time.

I don't think it makes sense to pessimize bare-metal uses simply because
there's a qemu-user limitation.

Thanks,
Florian