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1 | Why the "volatile" type class should not be used | |
2 | ------------------------------------------------ | |
3 | ||
4 | C programmers have often taken volatile to mean that the variable could be | |
5 | changed outside of the current thread of execution; as a result, they are | |
6 | sometimes tempted to use it in kernel code when shared data structures are | |
7 | being used. In other words, they have been known to treat volatile types | |
8 | as a sort of easy atomic variable, which they are not. The use of volatile in | |
9 | kernel code is almost never correct; this document describes why. | |
10 | ||
11 | The key point to understand with regard to volatile is that its purpose is | |
12 | to suppress optimization, which is almost never what one really wants to | |
13 | do. In the kernel, one must protect shared data structures against | |
14 | unwanted concurrent access, which is very much a different task. The | |
15 | process of protecting against unwanted concurrency will also avoid almost | |
16 | all optimization-related problems in a more efficient way. | |
17 | ||
18 | Like volatile, the kernel primitives which make concurrent access to data | |
19 | safe (spinlocks, mutexes, memory barriers, etc.) are designed to prevent | |
20 | unwanted optimization. If they are being used properly, there will be no | |
21 | need to use volatile as well. If volatile is still necessary, there is | |
22 | almost certainly a bug in the code somewhere. In properly-written kernel | |
23 | code, volatile can only serve to slow things down. | |
24 | ||
25 | Consider a typical block of kernel code: | |
26 | ||
27 | spin_lock(&the_lock); | |
28 | do_something_on(&shared_data); | |
29 | do_something_else_with(&shared_data); | |
30 | spin_unlock(&the_lock); | |
31 | ||
32 | If all the code follows the locking rules, the value of shared_data cannot | |
33 | change unexpectedly while the_lock is held. Any other code which might | |
34 | want to play with that data will be waiting on the lock. The spinlock | |
35 | primitives act as memory barriers - they are explicitly written to do so - | |
36 | meaning that data accesses will not be optimized across them. So the | |
37 | compiler might think it knows what will be in shared_data, but the | |
38 | spin_lock() call, since it acts as a memory barrier, will force it to | |
39 | forget anything it knows. There will be no optimization problems with | |
40 | accesses to that data. | |
41 | ||
42 | If shared_data were declared volatile, the locking would still be | |
43 | necessary. But the compiler would also be prevented from optimizing access | |
44 | to shared_data _within_ the critical section, when we know that nobody else | |
45 | can be working with it. While the lock is held, shared_data is not | |
46 | volatile. When dealing with shared data, proper locking makes volatile | |
47 | unnecessary - and potentially harmful. | |
48 | ||
49 | The volatile storage class was originally meant for memory-mapped I/O | |
50 | registers. Within the kernel, register accesses, too, should be protected | |
51 | by locks, but one also does not want the compiler "optimizing" register | |
52 | accesses within a critical section. But, within the kernel, I/O memory | |
53 | accesses are always done through accessor functions; accessing I/O memory | |
54 | directly through pointers is frowned upon and does not work on all | |
55 | architectures. Those accessors are written to prevent unwanted | |
56 | optimization, so, once again, volatile is unnecessary. | |
57 | ||
58 | Another situation where one might be tempted to use volatile is | |
59 | when the processor is busy-waiting on the value of a variable. The right | |
60 | way to perform a busy wait is: | |
61 | ||
62 | while (my_variable != what_i_want) | |
63 | cpu_relax(); | |
64 | ||
65 | The cpu_relax() call can lower CPU power consumption or yield to a | |
66 | hyperthreaded twin processor; it also happens to serve as a memory barrier, | |
67 | so, once again, volatile is unnecessary. Of course, busy-waiting is | |
68 | generally an anti-social act to begin with. | |
69 | ||
70 | There are still a few rare situations where volatile makes sense in the | |
71 | kernel: | |
72 | ||
73 | - The above-mentioned accessor functions might use volatile on | |
74 | architectures where direct I/O memory access does work. Essentially, | |
75 | each accessor call becomes a little critical section on its own and | |
76 | ensures that the access happens as expected by the programmer. | |
77 | ||
78 | - Inline assembly code which changes memory, but which has no other | |
79 | visible side effects, risks being deleted by GCC. Adding the volatile | |
80 | keyword to asm statements will prevent this removal. | |
81 | ||
82 | - The jiffies variable is special in that it can have a different value | |
83 | every time it is referenced, but it can be read without any special | |
84 | locking. So jiffies can be volatile, but the addition of other | |
85 | variables of this type is strongly frowned upon. Jiffies is considered | |
86 | to be a "stupid legacy" issue (Linus's words) in this regard; fixing it | |
87 | would be more trouble than it is worth. | |
88 | ||
89 | - Pointers to data structures in coherent memory which might be modified | |
90 | by I/O devices can, sometimes, legitimately be volatile. A ring buffer | |
91 | used by a network adapter, where that adapter changes pointers to | |
92 | indicate which descriptors have been processed, is an example of this | |
93 | type of situation. | |
94 | ||
95 | For most code, none of the above justifications for volatile apply. As a | |
96 | result, the use of volatile is likely to be seen as a bug and will bring | |
97 | additional scrutiny to the code. Developers who are tempted to use | |
98 | volatile should take a step back and think about what they are truly trying | |
99 | to accomplish. | |
100 | ||
101 | Patches to remove volatile variables are generally welcome - as long as | |
102 | they come with a justification which shows that the concurrency issues have | |
103 | been properly thought through. | |
104 | ||
105 | ||
106 | NOTES | |
107 | ----- | |
108 | ||
109 | [1] http://lwn.net/Articles/233481/ | |
110 | [2] http://lwn.net/Articles/233482/ | |
111 | ||
112 | CREDITS | |
113 | ------- | |
114 | ||
115 | Original impetus and research by Randy Dunlap | |
116 | Written by Jonathan Corbet | |
117 | Improvements via comments from Satyam Sharma, Johannes Stezenbach, Jesper | |
118 | Juhl, Heikki Orsila, H. Peter Anvin, Philipp Hahn, and Stefan | |
119 | Richter. |