Re: [PATCH v2 2/3] Y2038: nptl: Provide futex_abstimed_wait64 supporting 64 bit time

[Lukasz Majewski <lukma@denx.de>]

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Hi Adhemerval,

> On 30/09/2020 09:47, Lukasz Majewski wrote:
> > Hi Adhemerval,
> >   
> >> On 19/09/2020 10:07, Lukasz Majewski wrote:  
> >>> This is the helper function, which uses struct __timespec64
> >>> to provide 64 bit absolute time to futex syscalls.
> >>>
> >>> The aim of this function is to move convoluted pre-processor
> >>> macro code from sysdeps/nptl/lowlevellock-futex.h to C
> >>> function in futex-internal.c
> >>>
> >>> The futex_abstimed_wait64 function has been put into a separate
> >>> file on the purpose - to avoid issues apparent on the m68k
> >>> architecture related to small number of available registers (there
> >>> is not enough registers to put all necessary arguments in them if
> >>> the above function would be added to futex-internal.h with
> >>> __always_inline attribute).
> >>>
> >>> Additional precautions for m68k port have been taken - the
> >>> futex-internal.c file will be compiled with -fno-inline flag.    
> >>
> >> LGTM with a nit regarding style and a clarification about 
> >> '-fno-inline'.  
> > 
> > Please find the explanation below.
> >   
> >>  
> >>>
> >>> ---
> >>> Changes for v2:
> >>> - Handle the case when *abstime pointer is NULL
> >>> ---
> >>>  sysdeps/nptl/futex-internal.c         | 70
> >>> +++++++++++++++++++++++++++ sysdeps/nptl/futex-internal.h
> >>> | 6 +++ sysdeps/unix/sysv/linux/m68k/Makefile |  2 +
> >>>  3 files changed, 78 insertions(+)
> >>>
> >>> diff --git a/sysdeps/nptl/futex-internal.c
> >>> b/sysdeps/nptl/futex-internal.c index 3366aac162..3211b4c94f
> >>> 100644 --- a/sysdeps/nptl/futex-internal.c
> >>> +++ b/sysdeps/nptl/futex-internal.c
> >>> @@ -45,6 +45,29 @@ __futex_abstimed_wait_cancelable32 (unsigned
> >>> int* futex_word, abstime != NULL ? &ts32 : NULL,
> >>>                                    NULL /* Unused.  */,
> >>> FUTEX_BITSET_MATCH_ANY); }
> >>> +
> >>> +static int
> >>> +__futex_abstimed_wait32 (unsigned int* futex_word,
> >>> +                         unsigned int expected, clockid_t
> >>> clockid,
> >>> +                         const struct __timespec64* abstime,
> >>> +                         int private)
> >>> +{
> >>> +  struct timespec ts32;
> >>> +
> >>> +  if (abstime != NULL && ! in_time_t_range (abstime->tv_sec))
> >>> +    return -EOVERFLOW;
> >>> +
> >>> +  unsigned int clockbit = (clockid == CLOCK_REALTIME) ?
> >>> +    FUTEX_CLOCK_REALTIME : 0;
> >>> +  int op = __lll_private_flag (FUTEX_WAIT_BITSET | clockbit,
> >>> private); +
> >>> +  if (abstime != NULL)
> >>> +    ts32 = valid_timespec64_to_timespec (*abstime);
> >>> +
> >>> +  return INTERNAL_SYSCALL_CALL (futex, futex_word, op, expected,
> >>> +                                abstime != NULL ? &ts32 : NULL,
> >>> +                                NULL /* Unused.  */,
> >>> FUTEX_BITSET_MATCH_ANY); +}
> >>>  #endif
> >>>  
> >>>  int    
> >>
> >> Ok.
> >>  
> >>> @@ -97,3 +120,50 @@ __futex_abstimed_wait_cancelable64 (unsigned
> >>> int* futex_word, futex_fatal_error ();
> >>>      }
> >>>  }
> >>> +
> >>> +int
> >>> +__futex_abstimed_wait64 (unsigned int* futex_word, unsigned int
> >>> expected,
> >>> +                         clockid_t clockid,
> >>> +                         const struct __timespec64* abstime, int
> >>> private) +{
> >>> +  unsigned int clockbit;
> >>> +  int err;
> >>> +
> >>> +  /* Work around the fact that the kernel rejects negative
> >>> timeout values
> >>> +     despite them being valid.  */
> >>> +  if (__glibc_unlikely ((abstime != NULL) && (abstime->tv_sec <
> >>> 0)))
> >>> +    return ETIMEDOUT;
> >>> +
> >>> +  if (! lll_futex_supported_clockid(clockid))
> >>> +    return EINVAL;    
> >>
> >> Space before parentheses.   
> > 
> > Ok. Fixed.
> >   
> >>  
> >>> +
> >>> +  clockbit = (clockid == CLOCK_REALTIME) ? FUTEX_CLOCK_REALTIME :
> >>> 0;
> >>> +  int op = __lll_private_flag (FUTEX_WAIT_BITSET | clockbit,
> >>> private); +
> >>> +  err = INTERNAL_SYSCALL_CALL (futex_time64, futex_word, op,
> >>> expected,
> >>> +                               abstime, NULL /* Unused.  */,
> >>> +                               FUTEX_BITSET_MATCH_ANY);
> >>> +#ifndef __ASSUME_TIME64_SYSCALLS
> >>> +  if (err == -ENOSYS)
> >>> +    err = __futex_abstimed_wait32 (futex_word, expected,
> >>> +                                   clockid, abstime, private);
> >>> +#endif
> >>> +  switch (err)
> >>> +    {
> >>> +    case 0:
> >>> +    case -EAGAIN:
> >>> +    case -EINTR:
> >>> +    case -ETIMEDOUT:
> >>> +      return -err;
> >>> +
> >>> +    case -EFAULT: /* Must have been caused by a glibc or
> >>> application bug.  */
> >>> +    case -EINVAL: /* Either due to wrong alignment, unsupported
> >>> +		     clockid or due to the timeout not being
> >>> +		     normalized. Must have been caused by a glibc
> >>> or
> >>> +		     application bug.  */
> >>> +    case -ENOSYS: /* Must have been caused by a glibc bug.  */
> >>> +    /* No other errors are documented at this time.  */
> >>> +    default:
> >>> +      futex_fatal_error ();
> >>> +    }
> >>> +}    
> >>
> >> Ok.
> >>  
> >>> diff --git a/sysdeps/nptl/futex-internal.h
> >>> b/sysdeps/nptl/futex-internal.h index 7f3910ad98..1ba0d61938
> >>> 100644 --- a/sysdeps/nptl/futex-internal.h
> >>> +++ b/sysdeps/nptl/futex-internal.h
> >>> @@ -529,4 +529,10 @@ __futex_abstimed_wait_cancelable64 (unsigned
> >>> int* futex_word, const struct __timespec64* abstime,
> >>>                                      int private)
> >>> attribute_hidden; 
> >>> +int
> >>> +__futex_abstimed_wait64 (unsigned int* futex_word, unsigned int
> >>> expected,
> >>> +                         clockid_t clockid,
> >>> +                         const struct __timespec64* abstime,
> >>> +                         int private) attribute_hidden;
> >>> +
> >>>  #endif  /* futex-internal.h */    
> >>
> >> Ok.
> >>  
> >>> diff --git a/sysdeps/unix/sysv/linux/m68k/Makefile
> >>> b/sysdeps/unix/sysv/linux/m68k/Makefile index
> >>> be40fae68a..65164c5752 100644 ---
> >>> a/sysdeps/unix/sysv/linux/m68k/Makefile +++
> >>> b/sysdeps/unix/sysv/linux/m68k/Makefile @@ -21,3 +21,5 @@
> >>> sysdep-dl-routines += dl-static sysdep-others += lddlibc4
> >>>  install-bin += lddlibc4
> >>>  endif
> >>> +
> >>> +CFLAGS-futex-internal.c += -fno-inline
> >>>     
> >>
> >> Do we still need it?  
> > 
> > Unfortunately, yes. The m68k architecture has the issue with
> > properly compiling this code (in futex-internal.c) as it has very
> > limited number of registers (8 x 32 bit IIRC).
> > 
> > I did some investigation and it looks like static inline
> > in_time_t_range() function affects the compiler capabilities to
> > generate correct code.
> > 
> > It can be easily inlined on any architecture but m68k.
> > 
> > As m68k has issues with this code compilation - it is IMHO better to
> > disable inlining (-fno-inline) on this particular port.
> > 
> > As a result we would have slower execution only on relevant
> > functions, but 64 bit time support would be provided.   
> 
> I recall this was need on first patch for the cancellable 64-bit
> futex_abstimed_wait where it embedded the 32-bit fallback in the 
> __futex_abstimed_wait_cancelable64. And my suggestion to move it to 
> an external function seems to avoid the extra compiler flag.
> 
> This patchset uses the same strategy and I checked both patches
> and it seems that -fno-inline is not required to build m68k.

As fair as I can tell (at the time I tested it) - it was necessary to
have -fno-inline when this patch was applied on top of the one which is
now in the -master branch (in futex-internal.c).

I will double check it now with:

glibc/glibc-many-build --keep failed glibcs


Best regards,

Lukasz Majewski

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