"volatile considered harmful"

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

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[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.