Kernel code has to handle volatile data, but if you see the C keyword "volatile" in the kernel, it's a mistake. Here's why
Kernel code has to handle volatile data, but if you see the C keyword “volatile” in the kernel, it’s a mistake. Here’s why.
Brian Kernighan and Dennis Ritchie’s 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 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 on the topic and suggested to the author that he could maybe help “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 shared_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.
(Thanks to Randy Dunlap for getting things started and researching the issue, and to Satyam Sharma, and Johannes Stezenbach for comments on the first draft of this article).
