1 /* 2 * Copyright (C) 2008 Oracle. All rights reserved. 3 * 4 * This program is free software; you can redistribute it and/or 5 * modify it under the terms of the GNU General Public 6 * License v2 as published by the Free Software Foundation. 7 * 8 * This program is distributed in the hope that it will be useful, 9 * but WITHOUT ANY WARRANTY; without even the implied warranty of 10 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 11 * General Public License for more details. 12 * 13 * You should have received a copy of the GNU General Public 14 * License along with this program; if not, write to the 15 * Free Software Foundation, Inc., 59 Temple Place - Suite 330, 16 * Boston, MA 021110-1307, USA. 17 */ 18 #include <linux/sched.h> 19 #include <linux/gfp.h> 20 #include <linux/pagemap.h> 21 #include <linux/spinlock.h> 22 #include <linux/page-flags.h> 23 #include <asm/bug.h> 24 #include "ctree.h" 25 #include "extent_io.h" 26 #include "locking.h" 27 28 static inline void spin_nested(struct extent_buffer *eb) 29 { 30 spin_lock(&eb->lock); 31 } 32 33 /* 34 * Setting a lock to blocking will drop the spinlock and set the 35 * flag that forces other procs who want the lock to wait. After 36 * this you can safely schedule with the lock held. 37 */ 38 void btrfs_set_lock_blocking(struct extent_buffer *eb) 39 { 40 if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) { 41 set_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags); 42 spin_unlock(&eb->lock); 43 } 44 /* exit with the spin lock released and the bit set */ 45 } 46 47 /* 48 * clearing the blocking flag will take the spinlock again. 49 * After this you can't safely schedule 50 */ 51 void btrfs_clear_lock_blocking(struct extent_buffer *eb) 52 { 53 if (test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) { 54 spin_nested(eb); 55 clear_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags); 56 smp_mb__after_clear_bit(); 57 } 58 /* exit with the spin lock held */ 59 } 60 61 /* 62 * unfortunately, many of the places that currently set a lock to blocking 63 * don't end up blocking for every long, and often they don't block 64 * at all. For a dbench 50 run, if we don't spin one the blocking bit 65 * at all, the context switch rate can jump up to 400,000/sec or more. 66 * 67 * So, we're still stuck with this crummy spin on the blocking bit, 68 * at least until the most common causes of the short blocks 69 * can be dealt with. 70 */ 71 static int btrfs_spin_on_block(struct extent_buffer *eb) 72 { 73 int i; 74 for (i = 0; i < 512; i++) { 75 cpu_relax(); 76 if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) 77 return 1; 78 if (need_resched()) 79 break; 80 } 81 return 0; 82 } 83 84 /* 85 * This is somewhat different from trylock. It will take the 86 * spinlock but if it finds the lock is set to blocking, it will 87 * return without the lock held. 88 * 89 * returns 1 if it was able to take the lock and zero otherwise 90 * 91 * After this call, scheduling is not safe without first calling 92 * btrfs_set_lock_blocking() 93 */ 94 int btrfs_try_spin_lock(struct extent_buffer *eb) 95 { 96 int i; 97 98 spin_nested(eb); 99 if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) 100 return 1; 101 spin_unlock(&eb->lock); 102 103 /* spin for a bit on the BLOCKING flag */ 104 for (i = 0; i < 2; i++) { 105 if (!btrfs_spin_on_block(eb)) 106 break; 107 108 spin_nested(eb); 109 if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) 110 return 1; 111 spin_unlock(&eb->lock); 112 } 113 return 0; 114 } 115 116 /* 117 * the autoremove wake function will return 0 if it tried to wake up 118 * a process that was already awake, which means that process won't 119 * count as an exclusive wakeup. The waitq code will continue waking 120 * procs until it finds one that was actually sleeping. 121 * 122 * For btrfs, this isn't quite what we want. We want a single proc 123 * to be notified that the lock is ready for taking. If that proc 124 * already happen to be awake, great, it will loop around and try for 125 * the lock. 126 * 127 * So, btrfs_wake_function always returns 1, even when the proc that we 128 * tried to wake up was already awake. 129 */ 130 static int btrfs_wake_function(wait_queue_t *wait, unsigned mode, 131 int sync, void *key) 132 { 133 autoremove_wake_function(wait, mode, sync, key); 134 return 1; 135 } 136 137 /* 138 * returns with the extent buffer spinlocked. 139 * 140 * This will spin and/or wait as required to take the lock, and then 141 * return with the spinlock held. 142 * 143 * After this call, scheduling is not safe without first calling 144 * btrfs_set_lock_blocking() 145 */ 146 int btrfs_tree_lock(struct extent_buffer *eb) 147 { 148 DEFINE_WAIT(wait); 149 wait.func = btrfs_wake_function; 150 151 while(1) { 152 spin_nested(eb); 153 154 /* nobody is blocking, exit with the spinlock held */ 155 if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) 156 return 0; 157 158 /* 159 * we have the spinlock, but the real owner is blocking. 160 * wait for them 161 */ 162 spin_unlock(&eb->lock); 163 164 /* 165 * spin for a bit, and if the blocking flag goes away, 166 * loop around 167 */ 168 if (btrfs_spin_on_block(eb)) 169 continue; 170 171 prepare_to_wait_exclusive(&eb->lock_wq, &wait, 172 TASK_UNINTERRUPTIBLE); 173 174 if (test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) 175 schedule(); 176 177 finish_wait(&eb->lock_wq, &wait); 178 } 179 return 0; 180 } 181 182 /* 183 * Very quick trylock, this does not spin or schedule. It returns 184 * 1 with the spinlock held if it was able to take the lock, or it 185 * returns zero if it was unable to take the lock. 186 * 187 * After this call, scheduling is not safe without first calling 188 * btrfs_set_lock_blocking() 189 */ 190 int btrfs_try_tree_lock(struct extent_buffer *eb) 191 { 192 if (spin_trylock(&eb->lock)) { 193 if (test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) { 194 /* 195 * we've got the spinlock, but the real owner is 196 * blocking. Drop the spinlock and return failure 197 */ 198 spin_unlock(&eb->lock); 199 return 0; 200 } 201 return 1; 202 } 203 /* someone else has the spinlock giveup */ 204 return 0; 205 } 206 207 int btrfs_tree_unlock(struct extent_buffer *eb) 208 { 209 /* 210 * if we were a blocking owner, we don't have the spinlock held 211 * just clear the bit and look for waiters 212 */ 213 if (test_and_clear_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) 214 smp_mb__after_clear_bit(); 215 else 216 spin_unlock(&eb->lock); 217 218 if (waitqueue_active(&eb->lock_wq)) 219 wake_up(&eb->lock_wq); 220 return 0; 221 } 222 223 void btrfs_assert_tree_locked(struct extent_buffer *eb) 224 { 225 if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) 226 assert_spin_locked(&eb->lock); 227 } 228