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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	[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.
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