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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.
+
+atomic_xchg requires 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.
+
+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.