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author	Dave Brolley <brolley@redhat.com>	2009-12-18 12:16:33 -0500
committer	Dave Brolley <brolley@redhat.com>	2009-12-18 12:16:33 -0500
commit	a18a40d0cf1408d5575165ebc4374a1ab4f9fbc0 (patch)
tree	60b3a851a23b3bd7d944604c931e8ef1a9bd23c7 /runtime/counter.c
parent	de23650e8c1d62114653427984e0b807c9f4df16 (diff)
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PR 10641: Do not allow -m when --unprivileged in the server.

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		Semantics and Behavior of Atomic and
		         Bitmask Operations

			  David S. Miller	 

	This document is intended to serve as a guide to Linux port
maintainers on how to implement atomic counter, bitops, and spinlock
interfaces properly.

	The atomic_t type should be defined as a signed integer.
Also, it should be made opaque such that any kind of cast to a normal
C integer type will fail.  Something like the following should
suffice:

	typedef struct { volatile int counter; } atomic_t;

Historically, counter has been declared volatile.  This is now discouraged.
See Documentation/volatile-considered-harmful.txt for the complete rationale.

local_t is very similar to atomic_t. If the counter is per CPU and only
updated by one CPU, local_t is probably more appropriate. Please see
Documentation/local_ops.txt for the semantics of local_t.

The first operations to implement for atomic_t's are the initializers and
plain reads.

	#define ATOMIC_INIT(i)		{ (i) }
	#define atomic_set(v, i)	((v)->counter = (i))

The first macro is used in definitions, such as:

static atomic_t my_counter = ATOMIC_INIT(1);

The initializer is atomic in that the return values of the atomic operations
are guaranteed to be correct reflecting the initialized value if the
initializer is used before runtime.  If the initializer is used at runtime, a
proper implicit or explicit read memory barrier is needed before reading the
value with atomic_read from another thread.

The second interface can be used at runtime, as in:

	struct foo { atomic_t counter; };
	...

	struct foo *k;

	k = kmalloc(sizeof(*k), GFP_KERNEL);
	if (!k)
		return -ENOMEM;
	atomic_set(&k->counter, 0);

The setting is atomic in that the return values of the atomic operations by
all threads are guaranteed to be correct reflecting either the value that has
been set with this operation or set with another operation.  A proper implicit
or explicit memory barrier is needed before the value set with the operation
is guaranteed to be readable with atomic_read from another thread.

Next, we have:

	#define atomic_read(v)	((v)->counter)

which simply reads the counter value currently visible to the calling thread.
The read is atomic in that the return value is guaranteed to be one of the
values initialized or modified with the interface operations if a proper
implicit or explicit memory barrier is used after possible runtime
initialization by any other thread and the value is modified only with the
interface operations.  atomic_read does not guarantee that the runtime
initialization by any other thread is visible yet, so the user of the
interface must take care of that with a proper implicit or explicit memory
barrier.

*** WARNING: atomic_read() and atomic_set() DO NOT IMPLY BARRIERS! ***

Some architectures may choose to use the volatile keyword, barriers, or inline
assembly to guarantee some degree of immediacy for atomic_read() and
atomic_set().  This is not uniformly guaranteed, and may change in the future,
so all users of atomic_t should treat atomic_read() and atomic_set() as simple
C statements that may be reordered or optimized away entirely by the compiler
or processor, and explicitly invoke the appropriate compiler and/or memory
barrier for each use case.  Failure to do so will result in code that may
suddenly break when used with different architectures or compiler
optimizations, or even changes in unrelated code which changes how the
compiler optimizes the section accessing atomic_t variables.

*** YOU HAVE BEEN WARNED! ***

Now, we move onto the atomic operation interfaces typically implemented with
the help of assembly code.

	void atomic_add(int i, atomic_t *v);
	void atomic_sub(int i, atomic_t *v);
	void atomic_inc(atomic_t *v);
	void atomic_dec(atomic_t *v);

These four routines add and subtract integral values to/from the given
atomic_t value.  The first two routines pass explicit integers by
which to make the adjustment, whereas the latter two use an implicit
adjustment value of "1".

One very important aspect of these two routines is that they DO NOT
require any explicit memory barriers.  They need only perform the
atomic_t counter update in an SMP safe manner.

Next, we have:

	int atomic_inc_return(atomic_t *v);
	int atomic_dec_return(atomic_t *v);

These routines add 1 and subtract 1, respectively, from the given


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