diff options
Diffstat (limited to 'Documentation/atomic_ops.txt')
| -rw-r--r-- | Documentation/atomic_ops.txt | 120 |
1 files changed, 101 insertions, 19 deletions
diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt index 3bd585b4492..68542fe13b8 100644 --- a/Documentation/atomic_ops.txt +++ b/Documentation/atomic_ops.txt @@ -84,6 +84,93 @@ 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. @@ -166,6 +253,8 @@ 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, @@ -196,15 +285,13 @@ 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); + void smp_mb__before_atomic(void); + void smp_mb__after_atomic(void); -For example, smp_mb__before_atomic_dec() can be used like so: +For example, smp_mb__before_atomic() can be used like so: obj->dead = 1; - smp_mb__before_atomic_dec(); + smp_mb__before_atomic(); atomic_dec(&obj->ref_count); It makes sure that all memory operations preceding the atomic_dec() @@ -213,15 +300,10 @@ 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 +Without the explicit smp_mb__before_atomic() 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 @@ -398,12 +480,12 @@ Finally there is the basic operation: 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: +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: - void smp_mb__before_clear_bit(void); - void smp_mb__after_clear_bit(void); + 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: @@ -411,13 +493,13 @@ brothers: /* All memory operations before this call will * be globally visible before the clear_bit(). */ - smp_mb__before_clear_bit(); + smp_mb__before_atomic(); clear_bit( ... ); /* The clear_bit() will be visible before all * subsequent memory operations. */ - smp_mb__after_clear_bit(); + smp_mb__after_atomic(); 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 |