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

Rev	Author	Line No.	Line
1111	jermar	1	/*
2682	jermar	2	* Copyright (c) 2008 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.
2682	jermar	44	* Because of non-atomic nature of operations performed sequentially on the
		45	* futex counter and the futex wait queue, there is a race condition:
1116	jermar	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
2682	jermar	57	* 7. A fixes futex->count (i.e. the number of waiting threads) by changing it
		58	* from 0 to 1
1116	jermar	59	*
		60	* Scenario 2 (conditional down operation vs. futex_up)
		61	* 1. assume wq->missed_wakeups == 0 && futex->count == 0
		62	* (i.e. thread A is in the critical section)
2682	jermar	63	* 2. thread B performs futex_trydown() operation and changes futex->count from
		64	* 0 to -1
1116	jermar	65	* B is now obliged to call SYS_FUTEX_SLEEP syscall
		66	* 3. A wants to leave the critical section and does futex_up()
2682	jermar	67	* 4. A thus changes futex->count from -1 to 0 and must call SYS_FUTEX_WAKEUP
		68	* syscall
1116	jermar	69	* 5. B finds the wait queue empty and immediatelly aborts the conditional sleep
2682	jermar	70	* 6. No thread is queueing in the wait queue so wq->missed_wakeups changes from
		71	* 0 to 1
		72	* 6. B fixes futex->count (i.e. the number of waiting threads) by changing it
		73	* from 0 to 1
1116	jermar	74	*
2682	jermar	75	* Both scenarios allow two threads to be in the critical section
		76	* simultaneously. One without kernel intervention and the other through
		77	* wq->missed_wakeups being 1.
1116	jermar	78	*
2682	jermar	79	* To mitigate this problem, futex_down_timeout() detects that the syscall
		80	* didn't sleep in the wait queue, fixes the futex counter and RETRIES the
		81	* whole operation again.
1116	jermar	82	*/
		83
		84	/** Initialize futex counter.
		85	*
2682	jermar	86	* @param futex Futex.
		87	* @param val Initialization value.
1116	jermar	88	*/
2682	jermar	89	void futex_initialize(futex_t *futex, int val)
1111	jermar	90	{
		91	atomic_set(futex, val);
		92	}
		93
2682	jermar	94	int futex_down(futex_t *futex)
1111	jermar	95	{
1503	jermar	96	return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE);
1116	jermar	97	}
		98
2682	jermar	99	int futex_trydown(futex_t *futex)
1116	jermar	100	{
2682	jermar	101	return futex_down_timeout(futex, SYNCH_NO_TIMEOUT,
		102	SYNCH_FLAGS_NON_BLOCKING);
1116	jermar	103	}
		104
		105	/** Try to down the futex.
		106	*
2682	jermar	107	* @param futex Futex.
		108	* @param usec Microseconds to wait. Zero value means sleep without
		109	* timeout.
		110	* @param flags Select mode of operation. See comment for
		111	* waitq_sleep_timeout().
1116	jermar	112	*
2682	jermar	113	* @return ENOENT if there is no such virtual address. One of
		114	* ESYNCH_OK_ATOMIC and ESYNCH_OK_BLOCKED on success or
		115	* ESYNCH_TIMEOUT if the lock was not acquired because of
		116	* a timeout or ESYNCH_WOULD_BLOCK if the operation could
		117	* not be carried out atomically (if requested so).
1116	jermar	118	*/
2682	jermar	119	int futex_down_timeout(futex_t *futex, uint32_t usec, int flags)
1116	jermar	120	{
		121	int rc;
1111	jermar	122
1116	jermar	123	while (atomic_predec(futex) < 0) {
2682	jermar	124	rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count,
		125	(sysarg_t) usec, (sysarg_t) flags);
1111	jermar	126
1116	jermar	127	switch (rc) {
		128	case ESYNCH_OK_ATOMIC:
		129	/*
2682	jermar	130	* Because of a race condition between timeout and
		131	* futex_up() and between conditional
		132	* futex_down_timeout() and futex_up(), we have to give
		133	* up and try again in this special case.
1116	jermar	134	*/
		135	atomic_inc(futex);
		136	break;
		137
		138	case ESYNCH_TIMEOUT:
		139	atomic_inc(futex);
		140	return ESYNCH_TIMEOUT;
		141	break;
		142
		143	case ESYNCH_WOULD_BLOCK:
		144	/*
2682	jermar	145	* The conditional down operation should be implemented
		146	* this way. The userspace-only variant tends to
		147	* accumulate missed wakeups in the kernel futex wait
		148	* queue.
1116	jermar	149	*/
		150	atomic_inc(futex);
		151	return ESYNCH_WOULD_BLOCK;
		152	break;
		153
		154	case ESYNCH_OK_BLOCKED:
		155	/*
		156	* Enter the critical section.
2682	jermar	157	* The futex counter has already been incremented for
		158	* us.
1116	jermar	159	*/
		160	return ESYNCH_OK_BLOCKED;
		161	break;
		162	default:
		163	return rc;
		164	}
		165	}
		166
		167	/*
		168	* Enter the critical section.
		169	*/
		170	return ESYNCH_OK_ATOMIC;
1111	jermar	171	}
		172
1116	jermar	173	/** Up the futex.
		174	*
2682	jermar	175	* @param futex Futex.
1116	jermar	176	*
2682	jermar	177	* @return ENOENT if there is no such virtual address. Otherwise
		178	* zero.
1116	jermar	179	*/
2682	jermar	180	int futex_up(futex_t *futex)
1111	jermar	181	{
1114	jermar	182	long val;
		183
		184	val = atomic_postinc(futex);
		185	if (val < 0)
1111	jermar	186	return __SYSCALL1(SYS_FUTEX_WAKEUP, (sysarg_t) &futex->count);
		187
		188	return 0;
		189	}
1653	cejka	190
1866	jermar	191	/** @}
1653	cejka	192	*/

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