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 (at) 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 58 struct 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 66 static Bool s_debug; 67 #else 68 static Bool s_debug = True; 69 #endif 70 71 static const HChar *get_sched_lock_name(void) 72 { 73 return "ticket lock"; 74 } 75 76 static 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 92 static void destroy_sched_lock(struct sched_lock *p) 93 { 94 INNER_REQUEST(ANNOTATE_RWLOCK_DESTROY(p)); 95 VG_(free)(p); 96 } 97 98 static 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 */ 114 static 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 */ 158 static 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 } 185 186 const 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
