1 2.. _volatile_considered_harmful: 3 4Why the "volatile" type class should not be used 5------------------------------------------------ 6 7C programmers have often taken volatile to mean that the variable could be 8changed outside of the current thread of execution; as a result, they are 9sometimes tempted to use it in kernel code when shared data structures are 10being used. In other words, they have been known to treat volatile types 11as a sort of easy atomic variable, which they are not. The use of volatile in 12kernel code is almost never correct; this document describes why. 13 14The key point to understand with regard to volatile is that its purpose is 15to suppress optimization, which is almost never what one really wants to 16do. In the kernel, one must protect shared data structures against 17unwanted concurrent access, which is very much a different task. The 18process of protecting against unwanted concurrency will also avoid almost 19all optimization-related problems in a more efficient way. 20 21Like volatile, the kernel primitives which make concurrent access to data 22safe (spinlocks, mutexes, memory barriers, etc.) are designed to prevent 23unwanted optimization. If they are being used properly, there will be no 24need to use volatile as well. If volatile is still necessary, there is 25almost certainly a bug in the code somewhere. In properly-written kernel 26code, volatile can only serve to slow things down. 27 28Consider a typical block of kernel code:: 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 35If all the code follows the locking rules, the value of shared_data cannot 36change unexpectedly while the_lock is held. Any other code which might 37want to play with that data will be waiting on the lock. The spinlock 38primitives act as memory barriers - they are explicitly written to do so - 39meaning that data accesses will not be optimized across them. So the 40compiler might think it knows what will be in shared_data, but the 41spin_lock() call, since it acts as a memory barrier, will force it to 42forget anything it knows. There will be no optimization problems with 43accesses to that data. 44 45If shared_data were declared volatile, the locking would still be 46necessary. But the compiler would also be prevented from optimizing access 47to shared_data _within_ the critical section, when we know that nobody else 48can be working with it. While the lock is held, shared_data is not 49volatile. When dealing with shared data, proper locking makes volatile 50unnecessary - and potentially harmful. 51 52The volatile storage class was originally meant for memory-mapped I/O 53registers. Within the kernel, register accesses, too, should be protected 54by locks, but one also does not want the compiler "optimizing" register 55accesses within a critical section. But, within the kernel, I/O memory 56accesses are always done through accessor functions; accessing I/O memory 57directly through pointers is frowned upon and does not work on all 58architectures. Those accessors are written to prevent unwanted 59optimization, so, once again, volatile is unnecessary. 60 61Another situation where one might be tempted to use volatile is 62when the processor is busy-waiting on the value of a variable. The right 63way to perform a busy wait is:: 64 65 while (my_variable != what_i_want) 66 cpu_relax(); 67 68The cpu_relax() call can lower CPU power consumption or yield to a 69hyperthreaded twin processor; it also happens to serve as a compiler 70barrier, so, once again, volatile is unnecessary. Of course, busy- 71waiting is generally an anti-social act to begin with. 72 73There are still a few rare situations where volatile makes sense in the 74kernel: 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 98For most code, none of the above justifications for volatile apply. As a 99result, the use of volatile is likely to be seen as a bug and will bring 100additional scrutiny to the code. Developers who are tempted to use 101volatile should take a step back and think about what they are truly trying 102to accomplish. 103 104Patches to remove volatile variables are generally welcome - as long as 105they come with a justification which shows that the concurrency issues have 106been properly thought through. 107 108 109References 110========== 111 112[1] http://lwn.net/Articles/233481/ 113 114[2] http://lwn.net/Articles/233482/ 115 116Credits 117======= 118 119Original impetus and research by Randy Dunlap 120 121Written by Jonathan Corbet 122 123Improvements via comments from Satyam Sharma, Johannes Stezenbach, Jesper 124Juhl, Heikki Orsila, H. Peter Anvin, Philipp Hahn, and Stefan 125Richter. 126