Root/
Source at commit cdde9cf73945d547acd3e96f9508c79e84ad0bf1 created 12 years 9 months ago. By Maarten ter Huurne, MMC: JZ4740: Added support for CPU frequency changing | |
---|---|
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 compiler |
67 | barrier, so, once again, volatile is unnecessary. Of course, busy- |
68 | waiting is 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. |
120 |
Branches:
ben-wpan
ben-wpan-stefan
javiroman/ks7010
jz-2.6.34
jz-2.6.34-rc5
jz-2.6.34-rc6
jz-2.6.34-rc7
jz-2.6.35
jz-2.6.36
jz-2.6.37
jz-2.6.38
jz-2.6.39
jz-3.0
jz-3.1
jz-3.11
jz-3.12
jz-3.13
jz-3.15
jz-3.16
jz-3.18-dt
jz-3.2
jz-3.3
jz-3.4
jz-3.5
jz-3.6
jz-3.6-rc2-pwm
jz-3.9
jz-3.9-clk
jz-3.9-rc8
jz47xx
jz47xx-2.6.38
master
Tags:
od-2011-09-04
od-2011-09-18
v2.6.34-rc5
v2.6.34-rc6
v2.6.34-rc7
v3.9