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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 { 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! ***
-
-Properly aligned pointers, longs, ints, and chars (and unsigned
-equivalents) may be atomically loaded from and stored to in the same
-sense as described for atomic_read() and atomic_set(). The ACCESS_ONCE()
-macro should be used to prevent the compiler from using 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:
-
- 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:
-
- tmp = a;
- if (a > 0)
- for (;;)
- 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:
-
- while (ACCESS_ONCE(a) < 0)
- do_something();
-
-Alternatively, you could place a barrier() call in the loop.
-
-For another example, consider the following code:
-
- tmp_a = a;
- do_something_with(tmp_a);
- do_something_else_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:
-
- tmp_a = a;
- do_something_with(tmp_a);
- tmp_a = a;
- do_something_else_with(tmp_a);
-
-This could fatally confuse your code if it expected the same value
-to be passed to do_something_with() and do_something_else_with().
-
-The compiler would be likely to manufacture this additional load if
-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:
-
- tmp_a = ACCESS_ONCE(a);
- do_something_with(tmp_a);
- do_something_else_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:
-
- if (a)
- b = 9;
- else
- b = 42;
-
-The compiler is within its rights to manufacture an additional store
-by transforming the above code into the following:
-
- b = 42;
- if (a)
- b = 9;
-
-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:
-
- if (a)
- ACCESS_ONCE(b) = 9;
- else
- ACCESS_ONCE(b) = 42;
-
-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: ACCESS_ONCE() DOES NOT IMPLY A BARRIER! ***
-
-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
-unless it fails (returns 0).
-
-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, 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;
-
- 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 its 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.