Re: [PATCH] Remove arm lowlevellock.c

[Will Newton <will dot newton at linaro dot org>]

Hi Bernie,

ARM patches can now be sent to libc-alpha as ARM has moved from ports
into the main tree.

I'm not sure if we still use libc-ports for HPPA patches...

On 28 April 2014 15:50, Bernie Ogden <bernie.ogden@linaro.org> wrote:
> lowlevellock.c for arm differs from the generic lowlevellock.c only in
> insignificant ways, so can be removed. Happily, this fixes BZ 15119
> (unnecessary busy loop in __lll_timedlock_wait on arm).
>
> The notable differences between the arm and generic implementations are:
>
> 1) arm __lll_timedlock_wait has a fast path out if futex has been set
> to 0 between since the function was called. This seems unlikely to
> happen very often, so it seems at worst harmless to lose this fast
> path.
>
> 2) Some function in arm's lowlevellock.c set futex to 2 if it was 1.
> The generic version always sets the futex to 2. As futex can only be
> 0, 1 or 2 on entry into these functions, the behaviour is equivalent.
> (If the futex manages to be 0 on entry then we've just lost another
> unlikely fast path out.)
>
> There are no test suite regressions.
>
> Note that hppa and sparc also have their own lowlevellock.c. I believe
> hppa can also be removed, so I'll send a separate patch for that
> shortly. sparc's seems to be genuinely needed as it uses a different
> locking structure.
>
> Also note that the analysis at
> https://sourceware.org/ml/libc-ports/2013-02/msg00021.html indicates a
> further locking performance bug to fix - I've got a partial patch for
> that which I can submit once I've finished testing.
>
> 2014-04-24  Bernard Ogden <bernie.ogden@linaro.org>
>
> [BZ #15119]
> * sysdeps/unix/sysv/linux/arm/nptl/lowlevellock.c: Remove file.
>
>
> diff --git a/sysdeps/unix/sysv/linux/arm/nptl/lowlevellock.c
> b/sysdeps/unix/sysv/linux/arm/nptl/lowlevellock.c
> deleted file mode 100644
> index 9603d7b..0000000
> --- a/sysdeps/unix/sysv/linux/arm/nptl/lowlevellock.c
> +++ /dev/null
> @@ -1,132 +0,0 @@
> -/* low level locking for pthread library.  Generic futex-using version.
> -   Copyright (C) 2003-2014 Free Software Foundation, Inc.
> -   This file is part of the GNU C Library.
> -
> -   The GNU C Library is free software; you can redistribute it and/or
> -   modify it under the terms of the GNU Lesser General Public
> -   License as published by the Free Software Foundation; either
> -   version 2.1 of the License, or (at your option) any later version.
> -
> -   The GNU C Library is distributed in the hope that it will be useful,
> -   but WITHOUT ANY WARRANTY; without even the implied warranty of
> -   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> -   Lesser General Public License for more details.
> -
> -   You should have received a copy of the GNU Lesser General Public
> -   License along with the GNU C Library.  If not, see
> -   <http://www.gnu.org/licenses/>.  */
> -
> -#include <errno.h>
> -#include <sysdep.h>
> -#include <lowlevellock.h>
> -#include <sys/time.h>
> -
> -void
> -__lll_lock_wait_private (int *futex)
> -{
> -  do
> -    {
> -      int oldval = atomic_compare_and_exchange_val_acq (futex, 2, 1);
> -      if (oldval != 0)
> - lll_futex_wait (futex, 2, LLL_PRIVATE);
> -    }
> -  while (atomic_compare_and_exchange_bool_acq (futex, 2, 0) != 0);
> -}
> -
> -
> -/* These functions don't get included in libc.so  */
> -#ifdef IS_IN_libpthread
> -void
> -__lll_lock_wait (int *futex, int private)
> -{
> -  do
> -    {
> -      int oldval = atomic_compare_and_exchange_val_acq (futex, 2, 1);
> -      if (oldval != 0)
> - lll_futex_wait (futex, 2, private);
> -    }
> -  while (atomic_compare_and_exchange_bool_acq (futex, 2, 0) != 0);
> -}
> -
> -
> -int
> -__lll_timedlock_wait (int *futex, const struct timespec *abstime, int private)
> -{
> -  struct timespec rt;
> -
> -  /* Reject invalid timeouts.  */
> -  if (abstime->tv_nsec < 0 || abstime->tv_nsec >= 1000000000)
> -    return EINVAL;
> -
> -  /* Upgrade the lock.  */
> -  if (atomic_exchange_acq (futex, 2) == 0)
> -    return 0;
> -
> -  do
> -    {
> -      struct timeval tv;
> -
> -      /* Get the current time.  */
> -      (void) __gettimeofday (&tv, NULL);
> -
> -      /* Compute relative timeout.  */
> -      rt.tv_sec = abstime->tv_sec - tv.tv_sec;
> -      rt.tv_nsec = abstime->tv_nsec - tv.tv_usec * 1000;
> -      if (rt.tv_nsec < 0)
> - {
> -  rt.tv_nsec += 1000000000;
> -  --rt.tv_sec;
> - }
> -
> -      /* Already timed out?  */
> -      if (rt.tv_sec < 0)
> - return ETIMEDOUT;
> -
> -      // XYZ: Lost the lock to check whether it was private.
> -      lll_futex_timed_wait (futex, 2, &rt, private);
> -    }
> -  while (atomic_compare_and_exchange_bool_acq (futex, 2, 0) != 0);
> -
> -  return 0;
> -}
> -
> -
> -int
> -__lll_timedwait_tid (int *tidp, const struct timespec *abstime)
> -{
> -  int tid;
> -
> -  if (abstime->tv_nsec < 0 || abstime->tv_nsec >= 1000000000)
> -    return EINVAL;
> -
> -  /* Repeat until thread terminated.  */
> -  while ((tid = *tidp) != 0)
> -    {
> -      struct timeval tv;
> -      struct timespec rt;
> -
> -      /* Get the current time.  */
> -      (void) __gettimeofday (&tv, NULL);
> -
> -      /* Compute relative timeout.  */
> -      rt.tv_sec = abstime->tv_sec - tv.tv_sec;
> -      rt.tv_nsec = abstime->tv_nsec - tv.tv_usec * 1000;
> -      if (rt.tv_nsec < 0)
> - {
> -  rt.tv_nsec += 1000000000;
> -  --rt.tv_sec;
> - }
> -
> -      /* Already timed out?  */
> -      if (rt.tv_sec < 0)
> - return ETIMEDOUT;
> -
> -      /* Wait until thread terminates.  */
> -      // XYZ: Lost the lock to check whether it was private.
> -      if (lll_futex_timed_wait (tidp, tid, &rt, LLL_SHARED) == -ETIMEDOUT)
> - return ETIMEDOUT;
> -    }
> -
> -  return 0;
> -}
> -#endif



-- 
Will Newton
Toolchain Working Group, Linaro