1
2/*--------------------------------------------------------------------*/
3/*--- Linux ticket lock implementation         ticket-lock-linux.c ---*/
4/*---                                                              ---*/
5/*--- Guarantees fair scheduling even if multiple threads are      ---*/
6/*--- runnable at the same time on a multicore system. Has been    ---*/
7/*--- observed to cause a slow-down compared to the generic        ---*/
8/*--- scheduler lock with CPU frequency scaling enabled. Makes     ---*/
9/*--- Valgrind slightly faster if CPU frequency scaling has been   ---*/
10/*--- disabled. See also http://bugs.kde.org/show_bug.cgi?id=270006---*/
11/*--------------------------------------------------------------------*/
12
13/*
14   This file is part of Valgrind, a dynamic binary instrumentation
15   framework.
16
17   Copyright (C) 2011-2017 Bart Van Assche <bvanassche@acm.org>.
18
19   This program is free software; you can redistribute it and/or
20   modify it under the terms of the GNU General Public License as
21   published by the Free Software Foundation; either version 2 of the
22   License, or (at your option) any later version.
23
24   This program is distributed in the hope that it will be useful, but
25   WITHOUT ANY WARRANTY; without even the implied warranty of
26   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
27   General Public License for more details.
28
29   You should have received a copy of the GNU General Public License
30   along with this program; if not, write to the Free Software
31   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
32   02111-1307, USA.
33
34   The GNU General Public License is contained in the file COPYING.
35*/
36
37#include "pub_core_basics.h"
38#include "pub_core_libcassert.h"
39#include "pub_core_libcbase.h"     // VG_(memset)()
40#include "pub_core_libcprint.h"
41#include "pub_core_syscall.h"
42#include "pub_core_vki.h"
43#include "pub_core_vkiscnums.h"    // __NR_futex
44#include "pub_core_libcproc.h"
45#include "pub_core_mallocfree.h"
46#include "pub_core_threadstate.h"
47#include "pub_core_inner.h"
48#if defined(ENABLE_INNER_CLIENT_REQUEST)
49#include "helgrind/helgrind.h"
50#endif
51#include "priv_sched-lock.h"
52#include "priv_sched-lock-impl.h"
53
54#define TL_FUTEX_COUNT_LOG2 4
55#define TL_FUTEX_COUNT (1U << TL_FUTEX_COUNT_LOG2)
56#define TL_FUTEX_MASK (TL_FUTEX_COUNT - 1)
57
58struct sched_lock {
59   volatile unsigned head;
60   volatile unsigned tail;
61   volatile unsigned futex[TL_FUTEX_COUNT];
62   int owner;
63};
64
65#if 1
66static Bool s_debug;
67#else
68static Bool s_debug = True;
69#endif
70
71static const HChar *get_sched_lock_name(void)
72{
73   return "ticket lock";
74}
75
76static struct sched_lock *create_sched_lock(void)
77{
78   struct sched_lock *p;
79
80   p = VG_(malloc)("sched_lock", sizeof(*p));
81
82   // The futex syscall requires that a futex takes four bytes.
83   vg_assert(sizeof(p->futex[0]) == 4);
84
85   VG_(memset)(p, 0, sizeof(*p));
86
87   INNER_REQUEST(ANNOTATE_RWLOCK_CREATE(p));
88   INNER_REQUEST(ANNOTATE_BENIGN_RACE_SIZED(&p->futex, sizeof(p->futex), ""));
89   return p;
90}
91
92static void destroy_sched_lock(struct sched_lock *p)
93{
94   INNER_REQUEST(ANNOTATE_RWLOCK_DESTROY(p));
95   VG_(free)(p);
96}
97
98static int get_sched_lock_owner(struct sched_lock *p)
99{
100   return p->owner;
101}
102
103/*
104 * Acquire ticket lock. Increment the tail of the queue and use the original
105 * value as the ticket value. Wait until the head of the queue equals the
106 * ticket value. The futex used to wait depends on the ticket value in order
107 * to avoid that all threads get woken up every time a ticket lock is
108 * released. That last effect is sometimes called the "thundering herd"
109 * effect.
110 *
111 * See also Nick Piggin, x86: FIFO ticket spinlocks, Linux kernel mailing list
112 * (http://lkml.org/lkml/2007/11/1/125) for more info.
113 */
114static void acquire_sched_lock(struct sched_lock *p)
115{
116   unsigned ticket, futex_value;
117   volatile unsigned *futex;
118   SysRes sres;
119
120   ticket = __sync_fetch_and_add(&p->tail, 1);
121   futex = &p->futex[ticket & TL_FUTEX_MASK];
122   if (s_debug)
123      VG_(printf)("[%d/%d] acquire: ticket %u\n", VG_(getpid)(),
124                  VG_(gettid)(), ticket);
125   for (;;) {
126      futex_value = *futex;
127      __sync_synchronize();
128      if (ticket == p->head)
129         break;
130      if (s_debug)
131         VG_(printf)("[%d/%d] acquire: ticket %u - waiting until"
132                     " futex[%ld] != %u\n", VG_(getpid)(),
133                     VG_(gettid)(), ticket, (long)(futex - p->futex),
134                     futex_value);
135      sres = VG_(do_syscall3)(__NR_futex, (UWord)futex,
136                              VKI_FUTEX_WAIT | VKI_FUTEX_PRIVATE_FLAG,
137                              futex_value);
138      if (sr_isError(sres) && sr_Err(sres) != VKI_EAGAIN) {
139         VG_(printf)("futex_wait() returned error code %lu\n", sr_Err(sres));
140         vg_assert(False);
141      }
142   }
143   __sync_synchronize();
144   INNER_REQUEST(ANNOTATE_RWLOCK_ACQUIRED(p, /*is_w*/1));
145   vg_assert(p->owner == 0);
146   p->owner = VG_(gettid)();
147}
148
149/*
150 * Release a ticket lock by incrementing the head of the queue. Only generate
151 * a thread wakeup signal if at least one thread is waiting. If the queue tail
152 * matches the wakeup_ticket value, no threads have to be woken up.
153 *
154 * Note: tail will only be read after head has been incremented since both are
155 * declared as volatile and since the __sync...() functions imply a memory
156 * barrier.
157 */
158static void release_sched_lock(struct sched_lock *p)
159{
160   unsigned wakeup_ticket, futex_value;
161   volatile unsigned *futex;
162   SysRes sres;
163
164   vg_assert(p->owner != 0);
165   p->owner = 0;
166   INNER_REQUEST(ANNOTATE_RWLOCK_RELEASED(p, /*is_w*/1));
167   wakeup_ticket = __sync_fetch_and_add(&p->head, 1) + 1;
168   if (p->tail != wakeup_ticket) {
169      futex = &p->futex[wakeup_ticket & TL_FUTEX_MASK];
170      futex_value = __sync_fetch_and_add(futex, 1);
171      if (s_debug)
172         VG_(printf)("[%d/%d] release: waking up ticket %u (futex[%ld] = %u)"
173                     "\n", VG_(getpid)(), VG_(gettid)(), wakeup_ticket,
174                     (long)(futex - p->futex), futex_value);
175      sres = VG_(do_syscall3)(__NR_futex, (UWord)futex,
176                              VKI_FUTEX_WAKE | VKI_FUTEX_PRIVATE_FLAG,
177                              0x7fffffff);
178      vg_assert(!sr_isError(sres));
179   } else {
180      if (s_debug)
181         VG_(printf)("[%d/%d] release: no thread is waiting for ticket %u\n",
182                     VG_(getpid)(), VG_(gettid)(), wakeup_ticket);
183   }
184}
185
186const struct sched_lock_ops ML_(linux_ticket_lock_ops) = {
187   .get_sched_lock_name  = get_sched_lock_name,
188   .create_sched_lock    = create_sched_lock,
189   .destroy_sched_lock   = destroy_sched_lock,
190   .get_sched_lock_owner = get_sched_lock_owner,
191   .acquire_sched_lock   = acquire_sched_lock,
192   .release_sched_lock   = release_sched_lock,
193};
194