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2 | .. _volatile_considered_harmful: | |
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4 | Why the "volatile" type class should not be used |
5 | ------------------------------------------------ | |
6 | ||
7 | C programmers have often taken volatile to mean that the variable could be | |
8 | changed outside of the current thread of execution; as a result, they are | |
9 | sometimes tempted to use it in kernel code when shared data structures are | |
10 | being used. In other words, they have been known to treat volatile types | |
11 | as a sort of easy atomic variable, which they are not. The use of volatile in | |
12 | kernel code is almost never correct; this document describes why. | |
13 | ||
14 | The key point to understand with regard to volatile is that its purpose is | |
15 | to suppress optimization, which is almost never what one really wants to | |
16 | do. In the kernel, one must protect shared data structures against | |
17 | unwanted concurrent access, which is very much a different task. The | |
18 | process of protecting against unwanted concurrency will also avoid almost | |
19 | all optimization-related problems in a more efficient way. | |
20 | ||
21 | Like volatile, the kernel primitives which make concurrent access to data | |
22 | safe (spinlocks, mutexes, memory barriers, etc.) are designed to prevent | |
23 | unwanted optimization. If they are being used properly, there will be no | |
24 | need to use volatile as well. If volatile is still necessary, there is | |
25 | almost certainly a bug in the code somewhere. In properly-written kernel | |
26 | code, volatile can only serve to slow things down. | |
27 | ||
9c27d77d | 28 | Consider a typical block of kernel code:: |
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29 | |
30 | spin_lock(&the_lock); | |
31 | do_something_on(&shared_data); | |
32 | do_something_else_with(&shared_data); | |
33 | spin_unlock(&the_lock); | |
34 | ||
35 | If all the code follows the locking rules, the value of shared_data cannot | |
36 | change unexpectedly while the_lock is held. Any other code which might | |
37 | want to play with that data will be waiting on the lock. The spinlock | |
38 | primitives act as memory barriers - they are explicitly written to do so - | |
39 | meaning that data accesses will not be optimized across them. So the | |
40 | compiler might think it knows what will be in shared_data, but the | |
41 | spin_lock() call, since it acts as a memory barrier, will force it to | |
42 | forget anything it knows. There will be no optimization problems with | |
43 | accesses to that data. | |
44 | ||
45 | If shared_data were declared volatile, the locking would still be | |
46 | necessary. But the compiler would also be prevented from optimizing access | |
47 | to shared_data _within_ the critical section, when we know that nobody else | |
48 | can be working with it. While the lock is held, shared_data is not | |
49 | volatile. When dealing with shared data, proper locking makes volatile | |
50 | unnecessary - and potentially harmful. | |
51 | ||
52 | The volatile storage class was originally meant for memory-mapped I/O | |
53 | registers. Within the kernel, register accesses, too, should be protected | |
54 | by locks, but one also does not want the compiler "optimizing" register | |
55 | accesses within a critical section. But, within the kernel, I/O memory | |
56 | accesses are always done through accessor functions; accessing I/O memory | |
57 | directly through pointers is frowned upon and does not work on all | |
58 | architectures. Those accessors are written to prevent unwanted | |
59 | optimization, so, once again, volatile is unnecessary. | |
60 | ||
61 | Another situation where one might be tempted to use volatile is | |
62 | when the processor is busy-waiting on the value of a variable. The right | |
9c27d77d | 63 | way to perform a busy wait is:: |
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64 | |
65 | while (my_variable != what_i_want) | |
66 | cpu_relax(); | |
67 | ||
68 | The cpu_relax() call can lower CPU power consumption or yield to a | |
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69 | hyperthreaded twin processor; it also happens to serve as a compiler |
70 | barrier, so, once again, volatile is unnecessary. Of course, busy- | |
71 | waiting is generally an anti-social act to begin with. | |
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72 | |
73 | There are still a few rare situations where volatile makes sense in the | |
74 | kernel: | |
75 | ||
76 | - The above-mentioned accessor functions might use volatile on | |
77 | architectures where direct I/O memory access does work. Essentially, | |
78 | each accessor call becomes a little critical section on its own and | |
79 | ensures that the access happens as expected by the programmer. | |
80 | ||
81 | - Inline assembly code which changes memory, but which has no other | |
82 | visible side effects, risks being deleted by GCC. Adding the volatile | |
83 | keyword to asm statements will prevent this removal. | |
84 | ||
85 | - The jiffies variable is special in that it can have a different value | |
86 | every time it is referenced, but it can be read without any special | |
87 | locking. So jiffies can be volatile, but the addition of other | |
88 | variables of this type is strongly frowned upon. Jiffies is considered | |
89 | to be a "stupid legacy" issue (Linus's words) in this regard; fixing it | |
90 | would be more trouble than it is worth. | |
91 | ||
92 | - Pointers to data structures in coherent memory which might be modified | |
93 | by I/O devices can, sometimes, legitimately be volatile. A ring buffer | |
94 | used by a network adapter, where that adapter changes pointers to | |
95 | indicate which descriptors have been processed, is an example of this | |
96 | type of situation. | |
97 | ||
98 | For most code, none of the above justifications for volatile apply. As a | |
99 | result, the use of volatile is likely to be seen as a bug and will bring | |
100 | additional scrutiny to the code. Developers who are tempted to use | |
101 | volatile should take a step back and think about what they are truly trying | |
102 | to accomplish. | |
103 | ||
104 | Patches to remove volatile variables are generally welcome - as long as | |
105 | they come with a justification which shows that the concurrency issues have | |
106 | been properly thought through. | |
107 | ||
108 | ||
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109 | References |
110 | ========== | |
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111 | |
112 | [1] http://lwn.net/Articles/233481/ | |
9c27d77d | 113 | |
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114 | [2] http://lwn.net/Articles/233482/ |
115 | ||
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116 | Credits |
117 | ======= | |
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118 | |
119 | Original impetus and research by Randy Dunlap | |
9c27d77d | 120 | |
0faa4548 | 121 | Written by Jonathan Corbet |
9c27d77d | 122 | |
d9195881 | 123 | Improvements via comments from Satyam Sharma, Johannes Stezenbach, Jesper |
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124 | Juhl, Heikki Orsila, H. Peter Anvin, Philipp Hahn, and Stefan |
125 | Richter. |