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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

atomic_t and return the new counter value after the operation is

performed.

Unlike the above routines, it is required that explicit memory

barriers are performed before and after the operation.  It must be

done such that all memory operations before and after the atomic

operation calls are strongly ordered with respect to the atomic

operation itself.

For example, it should behave as if a smp_mb() call existed both

before and after the atomic operation.

If the atomic instructions used in an implementation provide explicit

memory barrier semantics which satisfy the above requirements, that is

fine as well.

Let's move on:

        int atomic_add_return(int i, atomic_t *v);

        int atomic_sub_return(int i, atomic_t *v);

These behave just like atomic_{inc,dec}_return() except that an

explicit counter adjustment is given instead of the implicit "1".

This means that like atomic_{inc,dec}_return(), the memory barrier

semantics are required.

Next:

        int atomic_inc_and_test(atomic_t *v);

        int atomic_dec_and_test(atomic_t *v);

These two routines increment and decrement by 1, respectively, the

given atomic counter.  They return a boolean indicating whether the

resulting counter value was zero or not.

It requires explicit memory barrier semantics around the operation as

above.

        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".  It requires explicit

memory barrier semantics around the operation.

        int atomic_add_negative(int i, atomic_t *v);

The given increment is added to the given atomic counter value.  A

boolean is return which indicates whether the resulting counter value

is negative.  It requires explicit memory barrier semantics around the

operation.

Then:

        int atomic_xchg(atomic_t *v, int new);

This performs an atomic exchange operation on the atomic variable v, setting

the given new value.  It returns the old value that the atomic variable v had

just before 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.

atomic_cmpxchg requires explicit memory barriers around the operation.

The semantics for atomic_cmpxchg are the same as those defined for 'cas'

below.

Finally:

        int atomic_add_unless(atomic_t *v, int a, int u);

If the atomic value v is not equal to u, this function adds a to v, and

returns non zero. If v is equal to u then it returns zero. This is done as

an atomic operation.

atomic_add_unless requires explicit memory barriers around the operation.

atomic_inc_not_zero, equivalent to atomic_add_unless(v, 1, 0)

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:

        void smp_mb__before_atomic_dec(void);

        void smp_mb__after_atomic_dec(void);

        void smp_mb__before_atomic_inc(void);

        void smp_mb__after_atomic_inc(void);

For example, smp_mb__before_atomic_dec() can be used like so:

        obj->dead = 1;

        smp_mb__before_atomic_dec();

        atomic_dec(&obj->ref_count);

It makes sure that all memory operations preceding the atomic_dec()

call are strongly ordered with respect to the atomic counter

operation.  In the above example, it guarantees that the assignment of

"1" to obj->dead will be globally visible to other cpus before the

atomic counter decrement.

Without the explicit smp_mb__before_atomic_dec() call, the

implementation could legally allow the atomic counter update visible

to other cpus before the "obj->dead = 1;" assignment.

The other three interfaces listed are used to provide explicit

ordering with respect to memory operations after an atomic_dec() call

(smp_mb__after_atomic_dec()) and around atomic_inc() calls

(smp_mb__{before,after}_atomic_inc()).

A missing memory barrier in the cases where they are required by the

atomic_t implementation above can have disastrous results.  Here is

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)

{

        obj->active = 1;

        list_add(&obj->list);

}

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;

                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);

        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);

}

(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

before the atomic counter decrement is performed.

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:

        cpu 0                           cpu 1

        obj_poke()                      obj_timeout()

        obj = obj_list_peek();

        ... gains ref to obj, refcnt=2

                                        obj_list_del(obj);

                                        obj->active = 0 ...

                                        ... visibility delayed ...

                                        atomic_dec_and_test()

                                        ... refcnt drops to 1 ...

        atomic_dec_and_test()

        ... refcount drops to 0 ...

        obj_destroy()

        BUG() triggers since obj->active

        still seen as one

                                        obj->active update visibility occurs

With the memory barrier semantics required of the atomic_t operations

which return values, the above sequence of memory visibility can never

happen.  Specifically, in the above case the atomic_dec_and_test()

counter decrement would not become globally visible until the

obj->active update does.

As a historical note, 32-bit Sparc used to only allow usage of

24-bits of it's atomic_t type.  This was because it used 8 bits

as a spinlock for SMP safety.  Sparc32 lacked a "compare and swap"

type instruction.  However, 32-bit Sparc has since been moved over

to a "hash table of spinlocks" scheme, that allows the full 32-bit

counter to be realized.  Essentially, an array of spinlocks are

indexed into based upon the address of the atomic_t being operated

on, and that lock protects the atomic operation.  Parisc uses the

same scheme.

Another note is that the atomic_t operations returning values are

extremely slow on an old 386.

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.

        void set_bit(unsigned long nr, volatile unsigned long *addr);

        void clear_bit(unsigned long nr, volatile unsigned long *addr);

        void change_bit(unsigned long nr, volatile unsigned long *addr);

These routines set, clear, and change, respectively, the bit number

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.

        int test_and_set_bit(unsigned long nr, volatile unsigned long *addr);

        int test_and_clear_bit(unsigned long nr, volatile unsigned long *addr);

        int test_and_change_bit(unsigned long nr, volatile unsigned long *addr);

Like the above, except that these routines return a boolean which

indicates whether the changed bit was set _BEFORE_ the atomic bit

operation.

WARNING! It is incredibly important that the value be a boolean,

ie. "0" or "1".  Do not try to be fancy and save a few instructions by

declaring the above to return "long" and just returning something like

"old_val & mask" because that will not work.

For one thing, this return value gets truncated to int in many code

paths using these interfaces, so on 64-bit if the bit is set in the

upper 32-bits then testers will never see that.

One great example of where this problem crops up are the thread_info

flag operations.  Routines such as test_and_set_ti_thread_flag() chop

the return value into an int.  There are other places where things

like this occur as well.

These routines, like the atomic_t counter operations returning values,

require explicit memory barrier semantics around their execution.  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:

        obj->dead = 1;

        if (test_and_set_bit(0, &obj->flags))

                /* ... */;

        obj->killed = 1;

The implementation of test_and_set_bit() must guarantee that

"obj->dead = 1;" is visible to cpus before the atomic memory operation

done by test_and_set_bit() becomes visible.  Likewise, the atomic

memory operation done by test_and_set_bit() must become visible before

"obj->killed = 1;" is visible.

Finally there is the basic operation:

        int test_bit(unsigned long nr, __const__ volatile unsigned long *addr);

Which returns a boolean indicating if bit "nr" is set in the bitmask

pointed to by "addr".

If explicit memory barriers are required around clear_bit() (which

does not return a value, and thus does not need to provide memory

barrier semantics), two interfaces are provided:

        void smp_mb__before_clear_bit(void);

        void smp_mb__after_clear_bit(void);

They are used as follows, and are akin to their atomic_t operation

brothers:

        /* All memory operations before this call will

         * be globally visible before the clear_bit().

         */

        smp_mb__before_clear_bit();

        clear_bit( ... );

        /* The clear_bit() will be visible before all

         * subsequent memory operations.

         */

         smp_mb__after_clear_bit();

There are two special bitops with lock barrier semantics (acquire/release,

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.

        int test_and_set_bit_lock(unsigned long nr, unsigned long *addr);

        void clear_bit_unlock(unsigned long nr, unsigned long *addr);

        void __clear_bit_unlock(unsigned long nr, unsigned long *addr);

The __clear_bit_unlock version is non-atomic, however it still implements

unlock barrier semantics. This can be useful if the lock itself is protecting

the other bits in the word.

Finally, there are non-atomic versions of the bitmask operations

provided.  They are used in contexts where some other higher-level SMP

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.

        void __set_bit(unsigned long nr, volatile unsigned long *addr);

        void __clear_bit(unsigned long nr, volatile unsigned long *addr);

        void __change_bit(unsigned long nr, volatile unsigned long *addr);

        int __test_and_set_bit(unsigned long nr, volatile unsigned long *addr);

        int __test_and_clear_bit(unsigned long nr, volatile unsigned long *addr);

        int __test_and_change_bit(unsigned long nr, volatile unsigned long *addr);

These non-atomic variants also do not require any special memory

barrier semantics.

The routines xchg() and cmpxchg() need the same exact memory barriers

as the atomic and bit operations returning values.

Spinlocks and rwlocks have memory barrier expectations as well.

The rule to follow is simple:

1) When acquiring a lock, the implementation must make it globally

   visible before any subsequent memory operation.

2) When releasing a lock, the implementation must make it such that

   all previous memory operations are globally visible before the

   lock release.

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.

        int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock);

Atomically decrement the given counter, and if will drop to zero

atomically acquire the given spinlock and perform the decrement

of the counter to zero.  If it does not drop to zero, do nothing

with the spinlock.

It is actually pretty simple to get the memory barrier correct.

Simply satisfy the spinlock grab requirements, which is make

sure the spinlock operation is globally visible before any

subsequent memory operation.

We can demonstrate this operation more clearly if we define

an abstract atomic operation:

        long cas(long *mem, long old, long new);

"cas" stands for "compare and swap".  It atomically:

1) Compares "old" with the value currently at "mem".

2) If they are equal, "new" is written to "mem".

3) Regardless, the current value at "mem" is returned.

As an example usage, here is what an atomic counter update

might look like:

void example_atomic_inc(long *counter)

{

        long old, new, ret;

        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;

                }

        }

        return went_to_zero;

}

Now, as far as memory barriers go, as long as spin_lock()

strictly orders all subsequent memory operations (including

the cas()) with respect to itself, things will be fine.

Said another way, _atomic_dec_and_lock() must guarantee that

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.