Commit 1b15611e authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar

arch,doc: Convert smp_mb__*()

Update the documentation to reflect the change of barrier primitives.
Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
Reviewed-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: default avatarDavid Howells <dhowells@redhat.com>
Link: http://lkml.kernel.org/n/tip-xslfehiga1twbk5uk94rij1e@git.kernel.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 09a01c0c
......@@ -285,15 +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()
......@@ -302,15 +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
......@@ -487,12 +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:
......@@ -500,13 +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
......
......@@ -1583,20 +1583,21 @@ There are some more advanced barrier functions:
insert anything more than a compiler barrier in a UP compilation.
(*) smp_mb__before_atomic_dec();
(*) smp_mb__after_atomic_dec();
(*) smp_mb__before_atomic_inc();
(*) smp_mb__after_atomic_inc();
(*) smp_mb__before_atomic();
(*) smp_mb__after_atomic();
These are for use with atomic add, subtract, increment and decrement
functions that don't return a value, especially when used for reference
counting. These functions do not imply memory barriers.
These are for use with atomic (such as add, subtract, increment and
decrement) functions that don't return a value, especially when used for
reference counting. These functions do not imply memory barriers.
These are also used for atomic bitop functions that do not return a
value (such as set_bit and clear_bit).
As an example, consider a piece of code that marks an object as being dead
and then decrements the object's reference count:
obj->dead = 1;
smp_mb__before_atomic_dec();
smp_mb__before_atomic();
atomic_dec(&obj->ref_count);
This makes sure that the death mark on the object is perceived to be set
......@@ -1606,27 +1607,6 @@ There are some more advanced barrier functions:
operations" subsection for information on where to use these.
(*) smp_mb__before_clear_bit(void);
(*) smp_mb__after_clear_bit(void);
These are for use similar to the atomic inc/dec barriers. These are
typically used for bitwise unlocking operations, so care must be taken as
there are no implicit memory barriers here either.
Consider implementing an unlock operation of some nature by clearing a
locking bit. The clear_bit() would then need to be barriered like this:
smp_mb__before_clear_bit();
clear_bit( ... );
This prevents memory operations before the clear leaking to after it. See
the subsection on "Locking Functions" with reference to RELEASE operation
implications.
See Documentation/atomic_ops.txt for more information. See the "Atomic
operations" subsection for information on where to use these.
MMIO WRITE BARRIER
------------------
......@@ -2283,11 +2263,11 @@ operations:
change_bit();
With these the appropriate explicit memory barrier should be used if necessary
(smp_mb__before_clear_bit() for instance).
(smp_mb__before_atomic() for instance).
The following also do _not_ imply memory barriers, and so may require explicit
memory barriers under some circumstances (smp_mb__before_atomic_dec() for
memory barriers under some circumstances (smp_mb__before_atomic() for
instance):
atomic_add();
......
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