[PATCH] "volatile considered harmful" document

[Jonathan Corbet <corbet@lwn.net>]

OK, here's an updated version of the volatile document - as a plain text
file this time.  It drops a new file in Documentation/, but might it be
better as an addition to CodingStyle?

Comments welcome,

jon

Tell kernel developers why they shouldn't use volatile.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>

diff -ruNp linux-2.6/Documentation/volatile.txt volatile/Documentation/volatile.txt
--- linux-2.6/Documentation/volatile.txt	1969-12-31 17:00:00.000000000 -0700
+++ volatile/Documentation/volatile.txt	2007-05-09 14:56:40.000000000 -0600
@@ -0,0 +1,127 @@
+Why the "volatile" type class should not be used
+------------------------------------------------
+
+The C Programming Language, Second Edition (copyright 1988, still known as
+"the new C book") has the following to say about the volatile keyword:
+
+	The purpose of volatile is to force an implementation to suppress
+	optimization that could otherwise occur.  For example, for a
+	machine with memory-mapped input/output, a pointer to a device
+	register might be declared as a pointer to volatile, in
+	order to prevent the compiler from removing apparently redundant
+	references through the pointer.
+
+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.  Andrew Morton recently called out[1] use of volatile in a
+submitted patch, saying:
+
+	The volatiles are a worry - volatile is said to be
+	basically-always-wrong in-kernel, although we've never managed to
+	document why, and i386 cheerfully uses it in readb() and friends.
+
+In response, Randy Dunlap pulled together some email from Linus[2] on the
+topic and suggested that we could maybe "document why."  Here is the
+result.
+
+The point that Linus often makes 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 accesses to data
+against race conditions, which is very much a different task.  
+
+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 some_data, but the
+spin_lock() call 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 _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.  This is why Linus says:
+
+	Also, more importantly, "volatile" is on the wrong _part_ of the
+	whole system. In C, it's "data" that is volatile, but that is
+	insane. Data isn't volatile - _accesses_ are volatile. So it may
+	make sense to say "make this particular _access_ be careful", but
+	not "make all accesses to this data use some random strategy".
+
+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 frowned upon.  Jiffies is considered to be a
+    "stupid legacy" issue in this regard.
+
+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.
+
+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 

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