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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/pagemap.h> |
20 | #include <linux/spinlock.h> |
21 | #include <linux/page-flags.h> |
22 | #include <asm/bug.h> |
23 | #include "ctree.h" |
24 | #include "extent_io.h" |
25 | #include "locking.h" |
26 | |
27 | static inline void spin_nested(struct extent_buffer *eb) |
28 | { |
29 | spin_lock(&eb->lock); |
30 | } |
31 | |
32 | /* |
33 | * Setting a lock to blocking will drop the spinlock and set the |
34 | * flag that forces other procs who want the lock to wait. After |
35 | * this you can safely schedule with the lock held. |
36 | */ |
37 | void btrfs_set_lock_blocking(struct extent_buffer *eb) |
38 | { |
39 | if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) { |
40 | set_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags); |
41 | spin_unlock(&eb->lock); |
42 | } |
43 | /* exit with the spin lock released and the bit set */ |
44 | } |
45 | |
46 | /* |
47 | * clearing the blocking flag will take the spinlock again. |
48 | * After this you can't safely schedule |
49 | */ |
50 | void btrfs_clear_lock_blocking(struct extent_buffer *eb) |
51 | { |
52 | if (test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) { |
53 | spin_nested(eb); |
54 | clear_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags); |
55 | smp_mb__after_clear_bit(); |
56 | } |
57 | /* exit with the spin lock held */ |
58 | } |
59 | |
60 | /* |
61 | * unfortunately, many of the places that currently set a lock to blocking |
62 | * don't end up blocking for very long, and often they don't block |
63 | * at all. For a dbench 50 run, if we don't spin on the blocking bit |
64 | * at all, the context switch rate can jump up to 400,000/sec or more. |
65 | * |
66 | * So, we're still stuck with this crummy spin on the blocking bit, |
67 | * at least until the most common causes of the short blocks |
68 | * can be dealt with. |
69 | */ |
70 | static int btrfs_spin_on_block(struct extent_buffer *eb) |
71 | { |
72 | int i; |
73 | |
74 | for (i = 0; i < 512; i++) { |
75 | if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) |
76 | return 1; |
77 | if (need_resched()) |
78 | break; |
79 | cpu_relax(); |
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 | if (btrfs_spin_on_block(eb)) { |
99 | spin_nested(eb); |
100 | if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) |
101 | return 1; |
102 | spin_unlock(&eb->lock); |
103 | } |
104 | /* spin for a bit on the BLOCKING flag */ |
105 | for (i = 0; i < 2; i++) { |
106 | cpu_relax(); |
107 | if (!btrfs_spin_on_block(eb)) |
108 | break; |
109 | |
110 | spin_nested(eb); |
111 | if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) |
112 | return 1; |
113 | spin_unlock(&eb->lock); |
114 | } |
115 | return 0; |
116 | } |
117 | |
118 | /* |
119 | * the autoremove wake function will return 0 if it tried to wake up |
120 | * a process that was already awake, which means that process won't |
121 | * count as an exclusive wakeup. The waitq code will continue waking |
122 | * procs until it finds one that was actually sleeping. |
123 | * |
124 | * For btrfs, this isn't quite what we want. We want a single proc |
125 | * to be notified that the lock is ready for taking. If that proc |
126 | * already happen to be awake, great, it will loop around and try for |
127 | * the lock. |
128 | * |
129 | * So, btrfs_wake_function always returns 1, even when the proc that we |
130 | * tried to wake up was already awake. |
131 | */ |
132 | static int btrfs_wake_function(wait_queue_t *wait, unsigned mode, |
133 | int sync, void *key) |
134 | { |
135 | autoremove_wake_function(wait, mode, sync, key); |
136 | return 1; |
137 | } |
138 | |
139 | /* |
140 | * returns with the extent buffer spinlocked. |
141 | * |
142 | * This will spin and/or wait as required to take the lock, and then |
143 | * return with the spinlock held. |
144 | * |
145 | * After this call, scheduling is not safe without first calling |
146 | * btrfs_set_lock_blocking() |
147 | */ |
148 | int btrfs_tree_lock(struct extent_buffer *eb) |
149 | { |
150 | DEFINE_WAIT(wait); |
151 | wait.func = btrfs_wake_function; |
152 | |
153 | if (!btrfs_spin_on_block(eb)) |
154 | goto sleep; |
155 | |
156 | while(1) { |
157 | spin_nested(eb); |
158 | |
159 | /* nobody is blocking, exit with the spinlock held */ |
160 | if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) |
161 | return 0; |
162 | |
163 | /* |
164 | * we have the spinlock, but the real owner is blocking. |
165 | * wait for them |
166 | */ |
167 | spin_unlock(&eb->lock); |
168 | |
169 | /* |
170 | * spin for a bit, and if the blocking flag goes away, |
171 | * loop around |
172 | */ |
173 | cpu_relax(); |
174 | if (btrfs_spin_on_block(eb)) |
175 | continue; |
176 | sleep: |
177 | prepare_to_wait_exclusive(&eb->lock_wq, &wait, |
178 | TASK_UNINTERRUPTIBLE); |
179 | |
180 | if (test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) |
181 | schedule(); |
182 | |
183 | finish_wait(&eb->lock_wq, &wait); |
184 | } |
185 | return 0; |
186 | } |
187 | |
188 | int btrfs_tree_unlock(struct extent_buffer *eb) |
189 | { |
190 | /* |
191 | * if we were a blocking owner, we don't have the spinlock held |
192 | * just clear the bit and look for waiters |
193 | */ |
194 | if (test_and_clear_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) |
195 | smp_mb__after_clear_bit(); |
196 | else |
197 | spin_unlock(&eb->lock); |
198 | |
199 | if (waitqueue_active(&eb->lock_wq)) |
200 | wake_up(&eb->lock_wq); |
201 | return 0; |
202 | } |
203 | |
204 | void btrfs_assert_tree_locked(struct extent_buffer *eb) |
205 | { |
206 | if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) |
207 | assert_spin_locked(&eb->lock); |
208 | } |
209 |
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Tags:
od-2011-09-04
od-2011-09-18
v2.6.34-rc5
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