From ed19993b5b0d05d62cc883571519a67dae481a14 Mon Sep 17 00:00:00 2001 From: Torvald Riegel Date: Wed, 25 May 2016 23:43:36 +0200 Subject: New condvar implementation that provides stronger ordering guarantees. This is a new implementation for condition variables, required after http://austingroupbugs.net/view.php?id=609 to fix bug 13165. In essence, we need to be stricter in which waiters a signal or broadcast is required to wake up; this couldn't be solved using the old algorithm. ISO C++ made a similar clarification, so this also fixes a bug in current libstdc++, for example. We can't use the old algorithm anymore because futexes do not guarantee to wake in FIFO order. Thus, when we wake, we can't simply let any waiter grab a signal, but we need to ensure that one of the waiters happening before the signal is woken up. This is something the previous algorithm violated (see bug 13165). There's another issue specific to condvars: ABA issues on the underlying futexes. Unlike mutexes that have just three states, or semaphores that have no tokens or a limited number of them, the state of a condvar is the *order* of the waiters. A waiter on a semaphore can grab a token whenever one is available; a condvar waiter must only consume a signal if it is eligible to do so as determined by the relative order of the waiter and the signal. Therefore, this new algorithm maintains two groups of waiters: Those eligible to consume signals (G1), and those that have to wait until previous waiters have consumed signals (G2). Once G1 is empty, G2 becomes the new G1. 64b counters are used to avoid ABA issues. This condvar doesn't yet use a requeue optimization (ie, on a broadcast, waking just one thread and requeueing all others on the futex of the mutex supplied by the program). I don't think doing the requeue is necessarily the right approach (but I haven't done real measurements yet): * If a program expects to wake many threads at the same time and make that scalable, a condvar isn't great anyway because of how it requires waiters to operate mutually exclusive (due to the mutex usage). Thus, a thundering herd problem is a scalability problem with or without the optimization. Using something like a semaphore might be more appropriate in such a case. * The scalability problem is actually at the mutex side; the condvar could help (and it tries to with the requeue optimization), but it should be the mutex who decides how that is done, and whether it is done at all. * Forcing all but one waiter into the kernel-side wait queue of the mutex prevents/avoids the use of lock elision on the mutex. Thus, it prevents the only cure against the underlying scalability problem inherent to condvars. * If condvars use short critical sections (ie, hold the mutex just to check a binary flag or such), which they should do ideally, then forcing all those waiter to proceed serially with kernel-based hand-off (ie, futex ops in the mutex' contended state, via the futex wait queues) will be less efficient than just letting a scalable mutex implementation take care of it. Our current mutex impl doesn't employ spinning at all, but if critical sections are short, spinning can be much better. * Doing the requeue stuff requires all waiters to always drive the mutex into the contended state. This leads to each waiter having to call futex_wake after lock release, even if this wouldn't be necessary. [BZ #13165] * nptl/pthread_cond_broadcast.c (__pthread_cond_broadcast): Rewrite to use new algorithm. * nptl/pthread_cond_destroy.c (__pthread_cond_destroy): Likewise. * nptl/pthread_cond_init.c (__pthread_cond_init): Likewise. * nptl/pthread_cond_signal.c (__pthread_cond_signal): Likewise. * nptl/pthread_cond_wait.c (__pthread_cond_wait): Likewise. (__pthread_cond_timedwait): Move here from pthread_cond_timedwait.c. (__condvar_confirm_wakeup, __condvar_cancel_waiting, __condvar_cleanup_waiting, __condvar_dec_grefs, __pthread_cond_wait_common): New. (__condvar_cleanup): Remove. * npt/pthread_condattr_getclock.c (pthread_condattr_getclock): Adapt. * npt/pthread_condattr_setclock.c (pthread_condattr_setclock): Likewise. * npt/pthread_condattr_getpshared.c (pthread_condattr_getpshared): Likewise. * npt/pthread_condattr_init.c (pthread_condattr_init): Likewise. * nptl/tst-cond1.c: Add comment. * nptl/tst-cond20.c (do_test): Adapt. * nptl/tst-cond22.c (do_test): Likewise. * sysdeps/aarch64/nptl/bits/pthreadtypes.h (pthread_cond_t): Adapt structure. * sysdeps/arm/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/ia64/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/m68k/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/microblaze/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/mips/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/nios2/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/s390/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/sh/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/tile/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/unix/sysv/linux/alpha/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/unix/sysv/linux/powerpc/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/x86/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/nptl/internaltypes.h (COND_NWAITERS_SHIFT): Remove. (COND_CLOCK_BITS): Adapt. * sysdeps/nptl/pthread.h (PTHREAD_COND_INITIALIZER): Adapt. * nptl/pthreadP.h (__PTHREAD_COND_CLOCK_MONOTONIC_MASK, __PTHREAD_COND_SHARED_MASK): New. * nptl/nptl-printers.py (CLOCK_IDS): Remove. (ConditionVariablePrinter, ConditionVariableAttributesPrinter): Adapt. * nptl/nptl_lock_constants.pysym: Adapt. * nptl/test-cond-printers.py: Adapt. * sysdeps/unix/sysv/linux/hppa/internaltypes.h (cond_compat_clear, cond_compat_check_and_clear): Adapt. * sysdeps/unix/sysv/linux/hppa/pthread_cond_timedwait.c: Remove file ... * sysdeps/unix/sysv/linux/hppa/pthread_cond_wait.c (__pthread_cond_timedwait): ... and move here. * nptl/DESIGN-condvar.txt: Remove file. * nptl/lowlevelcond.sym: Likewise. * nptl/pthread_cond_timedwait.c: Likewise. * sysdeps/unix/sysv/linux/i386/i486/pthread_cond_broadcast.S: Likewise. * sysdeps/unix/sysv/linux/i386/i486/pthread_cond_signal.S: Likewise. * sysdeps/unix/sysv/linux/i386/i486/pthread_cond_timedwait.S: Likewise. * sysdeps/unix/sysv/linux/i386/i486/pthread_cond_wait.S: Likewise. * sysdeps/unix/sysv/linux/i386/i586/pthread_cond_broadcast.S: Likewise. * sysdeps/unix/sysv/linux/i386/i586/pthread_cond_signal.S: Likewise. * sysdeps/unix/sysv/linux/i386/i586/pthread_cond_timedwait.S: Likewise. * sysdeps/unix/sysv/linux/i386/i586/pthread_cond_wait.S: Likewise. * sysdeps/unix/sysv/linux/i386/i686/pthread_cond_broadcast.S: Likewise. * sysdeps/unix/sysv/linux/i386/i686/pthread_cond_signal.S: Likewise. * sysdeps/unix/sysv/linux/i386/i686/pthread_cond_timedwait.S: Likewise. * sysdeps/unix/sysv/linux/i386/i686/pthread_cond_wait.S: Likewise. * sysdeps/unix/sysv/linux/x86_64/pthread_cond_broadcast.S: Likewise. * sysdeps/unix/sysv/linux/x86_64/pthread_cond_signal.S: Likewise. * sysdeps/unix/sysv/linux/x86_64/pthread_cond_timedwait.S: Likewise. * sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S: Likewise. --- .../sysv/linux/x86_64/pthread_cond_broadcast.S | 177 --------------------- 1 file changed, 177 deletions(-) delete mode 100644 sysdeps/unix/sysv/linux/x86_64/pthread_cond_broadcast.S (limited to 'sysdeps/unix/sysv/linux/x86_64/pthread_cond_broadcast.S') diff --git a/sysdeps/unix/sysv/linux/x86_64/pthread_cond_broadcast.S b/sysdeps/unix/sysv/linux/x86_64/pthread_cond_broadcast.S deleted file mode 100644 index de455dda4f..0000000000 --- a/sysdeps/unix/sysv/linux/x86_64/pthread_cond_broadcast.S +++ /dev/null @@ -1,177 +0,0 @@ -/* Copyright (C) 2002-2016 Free Software Foundation, Inc. - This file is part of the GNU C Library. - Contributed by Ulrich Drepper , 2002. - - 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 - . */ - -#include -#include -#include -#include -#include -#include -#include -#include - - .text - - /* int pthread_cond_broadcast (pthread_cond_t *cond) */ -ENTRY(__pthread_cond_broadcast) - - LIBC_PROBE (cond_broadcast, 1, %rdi) - - /* Get internal lock. */ - movl $1, %esi - xorl %eax, %eax - LOCK -#if cond_lock == 0 - cmpxchgl %esi, (%rdi) -#else - cmpxchgl %esi, cond_lock(%rdi) -#endif - jnz 1f - -2: addq $cond_futex, %rdi - movq total_seq-cond_futex(%rdi), %r9 - cmpq wakeup_seq-cond_futex(%rdi), %r9 - jna 4f - - /* Cause all currently waiting threads to recognize they are - woken up. */ - movq %r9, wakeup_seq-cond_futex(%rdi) - movq %r9, woken_seq-cond_futex(%rdi) - addq %r9, %r9 - movl %r9d, (%rdi) - incl broadcast_seq-cond_futex(%rdi) - - /* Get the address of the mutex used. */ - mov dep_mutex-cond_futex(%rdi), %R8_LP - - /* Unlock. */ - LOCK - decl cond_lock-cond_futex(%rdi) - jne 7f - -8: cmp $-1, %R8_LP - je 9f - - /* Do not use requeue for pshared condvars. */ - testl $PS_BIT, MUTEX_KIND(%r8) - jne 9f - - /* Requeue to a PI mutex if the PI bit is set. */ - movl MUTEX_KIND(%r8), %eax - andl $(ROBUST_BIT|PI_BIT), %eax - cmpl $PI_BIT, %eax - je 81f - - /* Wake up all threads. */ -#ifdef __ASSUME_PRIVATE_FUTEX - movl $(FUTEX_CMP_REQUEUE|FUTEX_PRIVATE_FLAG), %esi -#else - movl %fs:PRIVATE_FUTEX, %esi - orl $FUTEX_CMP_REQUEUE, %esi -#endif - movl $SYS_futex, %eax - movl $1, %edx - movl $0x7fffffff, %r10d - syscall - - /* For any kind of error, which mainly is EAGAIN, we try again - with WAKE. The general test also covers running on old - kernels. */ - cmpq $-4095, %rax - jae 9f - -10: xorl %eax, %eax - retq - - /* Wake up all threads. */ -81: movl $(FUTEX_CMP_REQUEUE_PI|FUTEX_PRIVATE_FLAG), %esi - movl $SYS_futex, %eax - movl $1, %edx - movl $0x7fffffff, %r10d - syscall - - /* For any kind of error, which mainly is EAGAIN, we try again - with WAKE. The general test also covers running on old - kernels. */ - cmpq $-4095, %rax - jb 10b - jmp 9f - - .align 16 - /* Unlock. */ -4: LOCK - decl cond_lock-cond_futex(%rdi) - jne 5f - -6: xorl %eax, %eax - retq - - /* Initial locking failed. */ -1: -#if cond_lock != 0 - addq $cond_lock, %rdi -#endif - LP_OP(cmp) $-1, dep_mutex-cond_lock(%rdi) - movl $LLL_PRIVATE, %eax - movl $LLL_SHARED, %esi - cmovne %eax, %esi - callq __lll_lock_wait -#if cond_lock != 0 - subq $cond_lock, %rdi -#endif - jmp 2b - - /* Unlock in loop requires wakeup. */ -5: addq $cond_lock-cond_futex, %rdi - LP_OP(cmp) $-1, dep_mutex-cond_lock(%rdi) - movl $LLL_PRIVATE, %eax - movl $LLL_SHARED, %esi - cmovne %eax, %esi - callq __lll_unlock_wake - jmp 6b - - /* Unlock in loop requires wakeup. */ -7: addq $cond_lock-cond_futex, %rdi - cmp $-1, %R8_LP - movl $LLL_PRIVATE, %eax - movl $LLL_SHARED, %esi - cmovne %eax, %esi - callq __lll_unlock_wake - subq $cond_lock-cond_futex, %rdi - jmp 8b - -9: /* The futex requeue functionality is not available. */ - cmp $-1, %R8_LP - movl $0x7fffffff, %edx -#ifdef __ASSUME_PRIVATE_FUTEX - movl $FUTEX_WAKE, %eax - movl $(FUTEX_WAKE|FUTEX_PRIVATE_FLAG), %esi - cmove %eax, %esi -#else - movl $0, %eax - movl %fs:PRIVATE_FUTEX, %esi - cmove %eax, %esi - orl $FUTEX_WAKE, %esi -#endif - movl $SYS_futex, %eax - syscall - jmp 10b -END(__pthread_cond_broadcast) - -versioned_symbol (libpthread, __pthread_cond_broadcast, pthread_cond_broadcast, - GLIBC_2_3_2) -- cgit 1.4.1