- Semantics and Behavior of Atomic and
- Bitmask Operations
+=======================================================
+Semantics and Behavior of Atomic and Bitmask Operations
+=======================================================
- David S. Miller
+:Author: David S. Miller
- This document is intended to serve as a guide to Linux port
+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 and
+Atomic Type And Operations
+==========================
+
+The atomic_t type should be defined as a signed integer and
the atomic_long_t type as a signed long integer. Also, they should
be made opaque such that any kind of cast to a normal C integer type
-will fail. Something like the following should suffice:
+will fail. Something like the following should suffice::
typedef struct { int counter; } atomic_t;
typedef struct { long counter; } atomic_long_t;
Historically, counter has been declared volatile. This is now discouraged.
-See Documentation/process/volatile-considered-harmful.rst for the complete rationale.
+See :ref:`Documentation/process/volatile-considered-harmful.rst
+<volatile_considered_harmful>` 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.
+:ref:`Documentation/core-api/local_ops.rst <local_ops>` for the semantics of
+local_t.
The first operations to implement for atomic_t's are the initializers and
-plain reads.
+plain reads. ::
#define ATOMIC_INIT(i) { (i) }
#define atomic_set(v, i) ((v)->counter = (i))
-The first macro is used in definitions, such as:
+The first macro is used in definitions, such as::
-static atomic_t my_counter = ATOMIC_INIT(1);
+ 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
proper implicit or explicit read memory barrier is needed before reading the
value with atomic_read from another thread.
-As with all of the atomic_ interfaces, replace the leading "atomic_"
-with "atomic_long_" to operate on atomic_long_t.
+As with all of the ``atomic_`` interfaces, replace the leading ``atomic_``
+with ``atomic_long_`` to operate on atomic_long_t.
-The second interface can be used at runtime, as in:
+The second interface can be used at runtime, as in::
struct foo { atomic_t counter; };
...
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:
+Next, we have::
#define atomic_read(v) ((v)->counter)
interface must take care of that with a proper implicit or explicit memory
barrier.
-*** WARNING: atomic_read() and atomic_set() DO NOT IMPLY BARRIERS! ***
+.. warning::
-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.
+ ``atomic_read()`` and ``atomic_set()`` DO NOT IMPLY BARRIERS!
-*** YOU HAVE BEEN WARNED! ***
+ 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.
Properly aligned pointers, longs, ints, and chars (and unsigned
equivalents) may be atomically loaded from and stored to in the same
optimizations that might otherwise optimize accesses out of existence on
the one hand, or that might create unsolicited accesses on the other.
-For example consider the following code:
+For example consider the following code::
while (a > 0)
do_something();
If the compiler can prove that do_something() does not store to the
variable a, then the compiler is within its rights transforming this to
-the following:
+the following::
tmp = a;
if (a > 0)
do_something();
If you don't want the compiler to do this (and you probably don't), then
-you should use something like the following:
+you should use something like the following::
while (READ_ONCE(a) < 0)
do_something();
Alternatively, you could place a barrier() call in the loop.
-For another example, consider the following code:
+For another example, consider the following code::
tmp_a = a;
do_something_with(tmp_a);
If the compiler can prove that do_something_with() does not store to the
variable a, then the compiler is within its rights to manufacture an
-additional load as follows:
+additional load as follows::
tmp_a = a;
do_something_with(tmp_a);
do_something_with() was an inline function that made very heavy use
of registers: reloading from variable a could save a flush to the
stack and later reload. To prevent the compiler from attacking your
-code in this manner, write the following:
+code in this manner, write the following::
tmp_a = READ_ONCE(a);
do_something_with(tmp_a);
For a final example, consider the following code, assuming that the
variable a is set at boot time before the second CPU is brought online
-and never changed later, so that memory barriers are not needed:
+and never changed later, so that memory barriers are not needed::
if (a)
b = 9;
b = 42;
The compiler is within its rights to manufacture an additional store
-by transforming the above code into the following:
+by transforming the above code into the following::
b = 42;
if (a)
This could come as a fatal surprise to other code running concurrently
that expected b to never have the value 42 if a was zero. To prevent
-the compiler from doing this, write something like:
+the compiler from doing this, write something like::
if (a)
WRITE_ONCE(b, 9);
Don't even -think- about doing this without proper use of memory barriers,
locks, or atomic operations if variable a can change at runtime!
-*** WARNING: READ_ONCE() OR WRITE_ONCE() DO NOT IMPLY A BARRIER! ***
+.. warning::
+
+ ``READ_ONCE()`` OR ``WRITE_ONCE()`` DO NOT IMPLY A BARRIER!
Now, we move onto the atomic operation interfaces typically implemented with
-the help of assembly code.
+the help of assembly code. ::
void atomic_add(int i, atomic_t *v);
void atomic_sub(int i, atomic_t *v);
require any explicit memory barriers. They need only perform the
atomic_t counter update in an SMP safe manner.
-Next, we have:
+Next, we have::
int atomic_inc_return(atomic_t *v);
int atomic_dec_return(atomic_t *v);
memory barrier semantics which satisfy the above requirements, that is
fine as well.
-Let's move on:
+Let's move on::
int atomic_add_return(int i, atomic_t *v);
int atomic_sub_return(int i, atomic_t *v);
This means that like atomic_{inc,dec}_return(), the memory barrier
semantics are required.
-Next:
+Next::
int atomic_inc_and_test(atomic_t *v);
int atomic_dec_and_test(atomic_t *v);
resulting counter value was zero or not.
Again, these primitives provide explicit memory barrier semantics around
-the atomic operation.
+the atomic operation::
int atomic_sub_and_test(int i, atomic_t *v);
This is identical to atomic_dec_and_test() except that an explicit
decrement is given instead of the implicit "1". This primitive must
-provide explicit memory barrier semantics around the operation.
+provide explicit memory barrier semantics around the operation::
int atomic_add_negative(int i, atomic_t *v);
This primitive must provide explicit memory barrier semantics around
the operation.
-Then:
+Then::
int atomic_xchg(atomic_t *v, int new);
the given new value. It returns the old value that the atomic variable v had
just before the operation.
-atomic_xchg must provide explicit memory barriers around the operation.
+atomic_xchg must provide explicit memory barriers around the operation. ::
int atomic_cmpxchg(atomic_t *v, int old, int new);
This performs an atomic compare exchange operation on the atomic value v,
with the given old and new values. Like all atomic_xxx operations,
atomic_cmpxchg will only satisfy its atomicity semantics as long as all
-other accesses of *v are performed through atomic_xxx operations.
+other accesses of \*v are performed through atomic_xxx operations.
atomic_cmpxchg must provide explicit memory barriers around the operation,
although if the comparison fails then no memory ordering guarantees are
The semantics for atomic_cmpxchg are the same as those defined for 'cas'
below.
-Finally:
+Finally::
int atomic_add_unless(atomic_t *v, int a, int u);
If a caller requires memory barrier semantics around an atomic_t
operation which does not return a value, a set of interfaces are
-defined which accomplish this:
+defined which accomplish this::
void smp_mb__before_atomic(void);
void smp_mb__after_atomic(void);
-For example, smp_mb__before_atomic() can be used like so:
+For example, smp_mb__before_atomic() can be used like so::
obj->dead = 1;
smp_mb__before_atomic();
an example, which follows a pattern occurring frequently in the Linux
kernel. It is the use of atomic counters to implement reference
counting, and it works such that once the counter falls to zero it can
-be guaranteed that no other entity can be accessing the object:
-
-static void obj_list_add(struct obj *obj, struct list_head *head)
-{
- obj->active = 1;
- list_add(&obj->list, head);
-}
-
-static void obj_list_del(struct obj *obj)
-{
- list_del(&obj->list);
- obj->active = 0;
-}
-
-static void obj_destroy(struct obj *obj)
-{
- BUG_ON(obj->active);
- kfree(obj);
-}
-
-struct obj *obj_list_peek(struct list_head *head)
-{
- if (!list_empty(head)) {
- struct obj *obj;
+be guaranteed that no other entity can be accessing the object::
+
+ static void obj_list_add(struct obj *obj, struct list_head *head)
+ {
+ obj->active = 1;
+ list_add(&obj->list, head);
+ }
+
+ static void obj_list_del(struct obj *obj)
+ {
+ list_del(&obj->list);
+ obj->active = 0;
+ }
- obj = list_entry(head->next, struct obj, list);
- atomic_inc(&obj->refcnt);
- return obj;
+ static void obj_destroy(struct obj *obj)
+ {
+ BUG_ON(obj->active);
+ kfree(obj);
}
- return NULL;
-}
-void obj_poke(void)
-{
- struct obj *obj;
+ struct obj *obj_list_peek(struct list_head *head)
+ {
+ if (!list_empty(head)) {
+ struct obj *obj;
+
+ obj = list_entry(head->next, struct obj, list);
+ atomic_inc(&obj->refcnt);
+ return obj;
+ }
+ return NULL;
+ }
+
+ void obj_poke(void)
+ {
+ struct obj *obj;
+
+ spin_lock(&global_list_lock);
+ obj = obj_list_peek(&global_list);
+ spin_unlock(&global_list_lock);
- spin_lock(&global_list_lock);
- obj = obj_list_peek(&global_list);
- spin_unlock(&global_list_lock);
+ if (obj) {
+ obj->ops->poke(obj);
+ if (atomic_dec_and_test(&obj->refcnt))
+ obj_destroy(obj);
+ }
+ }
+
+ void obj_timeout(struct obj *obj)
+ {
+ spin_lock(&global_list_lock);
+ obj_list_del(obj);
+ spin_unlock(&global_list_lock);
- if (obj) {
- obj->ops->poke(obj);
if (atomic_dec_and_test(&obj->refcnt))
obj_destroy(obj);
}
-}
-
-void obj_timeout(struct obj *obj)
-{
- spin_lock(&global_list_lock);
- obj_list_del(obj);
- spin_unlock(&global_list_lock);
- if (atomic_dec_and_test(&obj->refcnt))
- obj_destroy(obj);
-}
+.. note::
-(This is a simplification of the ARP queue management in the
- generic neighbour discover code of the networking. Olaf Kirch
- found a bug wrt. memory barriers in kfree_skb() that exposed
- the atomic_t memory barrier requirements quite clearly.)
+ This is a simplification of the ARP queue management in the generic
+ neighbour discover code of the networking. Olaf Kirch found a bug wrt.
+ memory barriers in kfree_skb() that exposed the atomic_t memory barrier
+ requirements quite clearly.
Given the above scheme, it must be the case that the obj->active
update done by the obj list deletion be visible to other processors
Otherwise, the counter could fall to zero, yet obj->active would still
be set, thus triggering the assertion in obj_destroy(). The error
-sequence looks like this:
+sequence looks like this::
cpu 0 cpu 1
obj_poke() obj_timeout()
Another note is that the atomic_t operations returning values are
extremely slow on an old 386.
+
+Atomic Bitmask
+==============
+
We will now cover the atomic bitmask operations. You will find that
their SMP and memory barrier semantics are similar in shape and scope
to the atomic_t ops above.
Native atomic bit operations are defined to operate on objects aligned
to the size of an "unsigned long" C data type, and are least of that
size. The endianness of the bits within each "unsigned long" are the
-native endianness of the cpu.
+native endianness of the cpu. ::
void set_bit(unsigned long nr, volatile unsigned long *addr);
void clear_bit(unsigned long nr, volatile unsigned long *addr);
indicated by "nr" on the bit mask pointed to by "ADDR".
They must execute atomically, yet there are no implicit memory barrier
-semantics required of these interfaces.
+semantics required of these interfaces. ::
int test_and_set_bit(unsigned long nr, volatile unsigned long *addr);
int test_and_clear_bit(unsigned long nr, volatile unsigned long *addr);
All memory operations before the atomic bit operation call must be
made visible globally before the atomic bit operation is made visible.
Likewise, the atomic bit operation must be visible globally before any
-subsequent memory operation is made visible. For example:
+subsequent memory operation is made visible. For example::
obj->dead = 1;
if (test_and_set_bit(0, &obj->flags))
memory operation done by test_and_set_bit() must become visible before
"obj->killed = 1;" is visible.
-Finally there is the basic operation:
+Finally there is the basic operation::
int test_bit(unsigned long nr, __const__ volatile unsigned long *addr);
If explicit memory barriers are required around {set,clear}_bit() (which do
not return a value, and thus does not need to provide memory barrier
-semantics), two interfaces are provided:
+semantics), two interfaces are provided::
void smp_mb__before_atomic(void);
void smp_mb__after_atomic(void);
They are used as follows, and are akin to their atomic_t operation
-brothers:
+brothers::
/* All memory operations before this call will
* be globally visible before the clear_bit().
same as spinlocks). These operate in the same way as their non-_lock/unlock
postfixed variants, except that they are to provide acquire/release semantics,
respectively. This means they can be used for bit_spin_trylock and
-bit_spin_unlock type operations without specifying any more barriers.
+bit_spin_unlock type operations without specifying any more barriers. ::
int test_and_set_bit_lock(unsigned long nr, unsigned long *addr);
void clear_bit_unlock(unsigned long nr, unsigned long *addr);
locking scheme is being used to protect the bitmask, and thus less
expensive non-atomic operations may be used in the implementation.
They have names similar to the above bitmask operation interfaces,
-except that two underscores are prefixed to the interface name.
+except that two underscores are prefixed to the interface name. ::
void __set_bit(unsigned long nr, volatile unsigned long *addr);
void __clear_bit(unsigned long nr, volatile unsigned long *addr);
memory-barrier semantics as the atomic and bit operations returning
values.
-Note: If someone wants to use xchg(), cmpxchg() and their variants,
-linux/atomic.h should be included rather than asm/cmpxchg.h, unless
-the code is in arch/* and can take care of itself.
+.. note::
+
+ If someone wants to use xchg(), cmpxchg() and their variants,
+ linux/atomic.h should be included rather than asm/cmpxchg.h, unless the
+ code is in arch/* and can take care of itself.
Spinlocks and rwlocks have memory barrier expectations as well.
The rule to follow is simple:
Which finally brings us to _atomic_dec_and_lock(). There is an
architecture-neutral version implemented in lib/dec_and_lock.c,
-but most platforms will wish to optimize this in assembler.
+but most platforms will wish to optimize this in assembler. ::
int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock);
subsequent memory operation.
We can demonstrate this operation more clearly if we define
-an abstract atomic operation:
+an abstract atomic operation::
long cas(long *mem, long old, long new);
3) Regardless, the current value at "mem" is returned.
As an example usage, here is what an atomic counter update
-might look like:
+might look like::
-void example_atomic_inc(long *counter)
-{
- long old, new, ret;
+ void example_atomic_inc(long *counter)
+ {
+ long old, new, ret;
- while (1) {
- old = *counter;
- new = old + 1;
+ while (1) {
+ old = *counter;
+ new = old + 1;
- ret = cas(counter, old, new);
- if (ret == old)
- break;
- }
-}
-
-Let's use cas() in order to build a pseudo-C atomic_dec_and_lock():
-
-int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
-{
- long old, new, ret;
- int went_to_zero;
-
- went_to_zero = 0;
- while (1) {
- old = atomic_read(atomic);
- new = old - 1;
- if (new == 0) {
- went_to_zero = 1;
- spin_lock(lock);
- }
- ret = cas(atomic, old, new);
- if (ret == old)
- break;
- if (went_to_zero) {
- spin_unlock(lock);
- went_to_zero = 0;
+ ret = cas(counter, old, new);
+ if (ret == old)
+ break;
}
}
- return went_to_zero;
-}
+Let's use cas() in order to build a pseudo-C atomic_dec_and_lock()::
+
+ int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
+ {
+ long old, new, ret;
+ int went_to_zero;
+
+ went_to_zero = 0;
+ while (1) {
+ old = atomic_read(atomic);
+ new = old - 1;
+ if (new == 0) {
+ went_to_zero = 1;
+ spin_lock(lock);
+ }
+ ret = cas(atomic, old, new);
+ if (ret == old)
+ break;
+ if (went_to_zero) {
+ spin_unlock(lock);
+ went_to_zero = 0;
+ }
+ }
+
+ return went_to_zero;
+ }
Now, as far as memory barriers go, as long as spin_lock()
strictly orders all subsequent memory operations (including
a counter dropping to zero is never made visible before the
spinlock being acquired.
-Note that this also means that for the case where the counter
-is not dropping to zero, there are no memory ordering
-requirements.
+.. note::
+
+ Note that this also means that for the case where the counter is not
+ dropping to zero, there are no memory ordering requirements.
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