Based on kernel version 3.13. Page generated on 2014-01-20 22:05 EST.
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  http://lwn.net/Articles/233481/ 110  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.