WebSVN – HelenOS – Blame – //trunk/uspace/lib/libc/generic/futex.c

Rev	Author	Line No.	Line
1111	jermar	1	/*
2071	jermar	2	* Copyright (c) 2006 Jakub Jermar
1111	jermar	3	* All rights reserved.
		4	*
		5	* Redistribution and use in source and binary forms, with or without
		6	* modification, are permitted provided that the following conditions
		7	* are met:
		8	*
		9	* - Redistributions of source code must retain the above copyright
		10	* notice, this list of conditions and the following disclaimer.
		11	* - Redistributions in binary form must reproduce the above copyright
		12	* notice, this list of conditions and the following disclaimer in the
		13	* documentation and/or other materials provided with the distribution.
		14	* - The name of the author may not be used to endorse or promote products
		15	* derived from this software without specific prior written permission.
		16	*
		17	* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
		18	* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
		19	* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
		20	* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
		21	* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
		22	* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
		23	* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
		24	* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
		25	* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
		26	* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1653	cejka	27	*/
		28
1866	jermar	29	/** @addtogroup libc
1653	cejka	30	* @{
		31	*/
		32	/** @file
1111	jermar	33	*/
		34
		35	#include <futex.h>
		36	#include <atomic.h>
		37	#include <libc.h>
		38	#include <stdio.h>
2541	jermar	39	#include <sys/types.h>
1116	jermar	40	#include <kernel/synch/synch.h>
1111	jermar	41
1116	jermar	42	/*
		43	* Note about race conditions.
		44	* Because of non-atomic nature of operations performed sequentially on the futex
		45	* counter and the futex wait queue, there is a race condition:
		46	*
1152	jermar	47	* (wq->missed_wakeups == 1) && (futex->count = 1)
1116	jermar	48	*
		49	* Scenario 1 (wait queue timeout vs. futex_up()):
		50	* 1. assume wq->missed_wakeups == 0 && futex->count == -1
		51	* (ie. thread A sleeping, thread B in the critical section)
		52	* 2. A receives timeout and gets removed from the wait queue
		53	* 3. B wants to leave the critical section and calls futex_up()
		54	* 4. B thus changes futex->count from -1 to 0
		55	* 5. B has to call SYS_FUTEX_WAKEUP syscall to wake up the sleeping thread
		56	* 6. B finds the wait queue empty and changes wq->missed_wakeups from 0 to 1
		57	* 7. A fixes futex->count (i.e. the number of waiting threads) by changing it from 0 to 1
		58	*
		59	* Scenario 2 (conditional down operation vs. futex_up)
		60	* 1. assume wq->missed_wakeups == 0 && futex->count == 0
		61	* (i.e. thread A is in the critical section)
		62	* 2. thread B performs futex_trydown() operation and changes futex->count from 0 to -1
		63	* B is now obliged to call SYS_FUTEX_SLEEP syscall
		64	* 3. A wants to leave the critical section and does futex_up()
		65	* 4. A thus changes futex->count from -1 to 0 and must call SYS_FUTEX_WAKEUP syscall
		66	* 5. B finds the wait queue empty and immediatelly aborts the conditional sleep
		67	* 6. No thread is queueing in the wait queue so wq->missed_wakeups changes from 0 to 1
		68	* 6. B fixes futex->count (i.e. the number of waiting threads) by changing it from 0 to 1
		69	*
		70	* Both scenarios allow two threads to be in the critical section simultaneously.
		71	* One without kernel intervention and the other through wq->missed_wakeups being 1.
		72	*
		73	* To mitigate this problem, futex_down_timeout() detects that the syscall didn't sleep
		74	* in the wait queue, fixes the futex counter and RETRIES the whole operation again.
		75	*
		76	*/
		77
		78	/** Initialize futex counter.
		79	*
		80	* @param futex Futex.
		81	* @param val Initialization value.
		82	*/
1111	jermar	83	void futex_initialize(atomic_t *futex, int val)
		84	{
		85	atomic_set(futex, val);
		86	}
		87
		88	int futex_down(atomic_t *futex)
		89	{
1503	jermar	90	return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE);
1116	jermar	91	}
		92
		93	int futex_trydown(atomic_t *futex)
		94	{
1503	jermar	95	return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NON_BLOCKING);
1116	jermar	96	}
		97
		98	/** Try to down the futex.
		99	*
		100	* @param futex Futex.
		101	* @param usec Microseconds to wait. Zero value means sleep without timeout.
1503	jermar	102	* @param flags Select mode of operation. See comment for waitq_sleep_timeout().
1116	jermar	103	*
		104	* @return ENOENT if there is no such virtual address. One of ESYNCH_OK_ATOMIC
		105	* and ESYNCH_OK_BLOCKED on success or ESYNCH_TIMEOUT if the lock was
		106	* not acquired because of a timeout or ESYNCH_WOULD_BLOCK if the
		107	* operation could not be carried out atomically (if requested so).
		108	*/
1503	jermar	109	int futex_down_timeout(atomic_t *futex, uint32_t usec, int flags)
1116	jermar	110	{
		111	int rc;
1111	jermar	112
1116	jermar	113	while (atomic_predec(futex) < 0) {
1503	jermar	114	rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count, (sysarg_t) usec, (sysarg_t) flags);
1111	jermar	115
1116	jermar	116	switch (rc) {
		117	case ESYNCH_OK_ATOMIC:
		118	/*
		119	* Because of a race condition between timeout and futex_up()
		120	* and between conditional futex_down_timeout() and futex_up(),
		121	* we have to give up and try again in this special case.
		122	*/
		123	atomic_inc(futex);
		124	break;
		125
		126	case ESYNCH_TIMEOUT:
		127	atomic_inc(futex);
		128	return ESYNCH_TIMEOUT;
		129	break;
		130
		131	case ESYNCH_WOULD_BLOCK:
		132	/*
		133	* The conditional down operation should be implemented this way.
		134	* The userspace-only variant tends to accumulate missed wakeups
		135	* in the kernel futex wait queue.
		136	*/
		137	atomic_inc(futex);
		138	return ESYNCH_WOULD_BLOCK;
		139	break;
		140
		141	case ESYNCH_OK_BLOCKED:
		142	/*
		143	* Enter the critical section.
		144	* The futex counter has already been incremented for us.
		145	*/
		146	return ESYNCH_OK_BLOCKED;
		147	break;
		148	default:
		149	return rc;
		150	}
		151	}
		152
		153	/*
		154	* Enter the critical section.
		155	*/
		156	return ESYNCH_OK_ATOMIC;
1111	jermar	157	}
		158
1116	jermar	159	/** Up the futex.
		160	*
		161	* @param futex Futex.
		162	*
1152	jermar	163	* @return ENOENT if there is no such virtual address. Otherwise zero.
1116	jermar	164	*/
1111	jermar	165	int futex_up(atomic_t *futex)
		166	{
1114	jermar	167	long val;
		168
		169	val = atomic_postinc(futex);
		170	if (val < 0)
1111	jermar	171	return __SYSCALL1(SYS_FUTEX_WAKEUP, (sysarg_t) &futex->count);
		172
		173	return 0;
		174	}
1653	cejka	175
1866	jermar	176	/** @}
1653	cejka	177	*/

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(root)//trunk/uspace/lib/libc/generic/futex.c – Rev 2541