Here's another version of the volatile document. Once again, I've tried
to address all of the comments. There haven't really been any recent
comments addressing the correctness of the document; people have been
more concerned with how it's expressed. I'm glad to see files in
Documentation/ held to a high standard of writing, but, unless somebody
has a factual issue this time around I would like to declare Mission
Accomplished and move on.
Thanks,
jon
---
Encourage developers to avoid the volatile type class in kernel code.
Signed-off-by: Jonathan Corbet <[email protected]>
diff --git a/Documentation/volatile-considered-harmful.txt b/Documentation/volatile-considered-harmful.txt
new file mode 100644
index 0000000..955c309
--- /dev/null
+++ b/Documentation/volatile-considered-harmful.txt
@@ -0,0 +1,119 @@
+Why the "volatile" type class should not be used
+------------------------------------------------
+
+C programmers have often taken volatile to mean that the variable could be
+changed outside of the current thread of execution; as a result, they are
+sometimes tempted to use it in kernel code when shared data structures are
+being used. In other words, they have been known to treat volatile types
+as a sort of easy atomic variable, which they are not. The use of volatile in
+kernel code is almost never correct; this document describes why.
+
+The key point to understand with regard to volatile is that its purpose is
+to suppress optimization, which is almost never what one really wants to
+do. In the kernel, one must protect shared data structures against
+unwanted concurrent access, which is very much a different task. The
+process of protecting against unwanted concurrency will also avoid almost
+all optimization-related problems in a more efficient way.
+
+Like volatile, the kernel primitives which make concurrent access to data
+safe (spinlocks, mutexes, memory barriers, etc.) are designed to prevent
+unwanted optimization. If they are being used properly, there will be no
+need to use volatile as well. If volatile is still necessary, there is
+almost certainly a bug in the code somewhere. In properly-written kernel
+code, volatile can only serve to slow things down.
+
+Consider a typical block of kernel code:
+
+ spin_lock(&the_lock);
+ do_something_on(&shared_data);
+ do_something_else_with(&shared_data);
+ spin_unlock(&the_lock);
+
+If all the code follows the locking rules, the value of shared_data cannot
+change unexpectedly while the_lock is held. Any other code which might
+want to play with that data will be waiting on the lock. The spinlock
+primitives act as memory barriers - they are explicitly written to do so -
+meaning that data accesses will not be optimized across them. So the
+compiler might think it knows what will be in shared_data, but the
+spin_lock() call, since it acts as a memory barrier, will force it to
+forget anything it knows. There will be no optimization problems with
+accesses to that data.
+
+If shared_data were declared volatile, the locking would still be
+necessary. But the compiler would also be prevented from optimizing access
+to shared_data _within_ the critical section, when we know that nobody else
+can be working with it. While the lock is held, shared_data is not
+volatile. When dealing with shared data, proper locking makes volatile
+unnecessary - and potentially harmful.
+
+The volatile storage class was originally meant for memory-mapped I/O
+registers. Within the kernel, register accesses, too, should be protected
+by locks, but one also does not want the compiler "optimizing" register
+accesses within a critical section. But, within the kernel, I/O memory
+accesses are always done through accessor functions; accessing I/O memory
+directly through pointers is frowned upon and does not work on all
+architectures. Those accessors are written to prevent unwanted
+optimization, so, once again, volatile is unnecessary.
+
+Another situation where one might be tempted to use volatile is
+when the processor is busy-waiting on the value of a variable. The right
+way to perform a busy wait is:
+
+ while (my_variable != what_i_want)
+ cpu_relax();
+
+The cpu_relax() call can lower CPU power consumption or yield to a
+hyperthreaded twin processor; it also happens to serve as a memory barrier,
+so, once again, volatile is unnecessary. Of course, busy-waiting is
+generally an anti-social act to begin with.
+
+There are still a few rare situations where volatile makes sense in the
+kernel:
+
+ - The above-mentioned accessor functions might use volatile on
+ architectures where direct I/O memory access does work. Essentially,
+ each accessor call becomes a little critical section on its own and
+ ensures that the access happens as expected by the programmer.
+
+ - Inline assembly code which changes memory, but which has no other
+ visible side effects, risks being deleted by GCC. Adding the volatile
+ keyword to asm statements will prevent this removal.
+
+ - The jiffies variable is special in that it can have a different value
+ every time it is referenced, but it can be read without any special
+ locking. So jiffies can be volatile, but the addition of other
+ variables of this type is strongly frowned upon. Jiffies is considered
+ to be a "stupid legacy" issue (Linus's words) in this regard; fixing it
+ would be more trouble than it is worth.
+
+ - Pointers to data structures in coherent memory which might be modified
+ by I/O devices can, sometimes, legitimately be volatile. A ring buffer
+ used by a network adapter, where that adapter changes pointers to
+ indicate which descriptors have been processed, is an example of this
+ type of situation.
+
+For most code, none of the above justifications for volatile apply. As a
+result, the use of volatile is likely to be seen as a bug and will bring
+additional scrutiny to the code. Developers who are tempted to use
+volatile should take a step back and think about what they are truly trying
+to accomplish.
+
+Patches to remove volatile variables are generally welcome - as long as
+they come with a justification which shows that the concurrency issues have
+been properly thought through.
+
+
+NOTES
+-----
+
+[1] http://lwn.net/Articles/233481/
+[2] http://lwn.net/Articles/233482/
+
+CREDITS
+-------
+
+Original impetus and research by Randy Dunlap
+Written by Jonathan Corbet
+Improvements via coments from Satyam Sharma, Johannes Stezenbach, Jesper
+ Juhl, Heikki Orsila, H. Peter Anvin, Philipp Hahn, and Stefan
+ Richter.
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