The LOCK and UNLOCK barriers as described in our barrier document are
generally known as ACQUIRE and RELEASE barriers in other literature.
Since we plan to introduce the acquire and release nomenclature in
generic kernel primitives we should ammend the document to avoid
confusion as to what an acquire/release means.
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
Cc: Michael Ellerman <michael@xxxxxxxxxxxxxx>
Cc: Michael Neuling <mikey@xxxxxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Victor Kaplansky <VICTORK@xxxxxxxxxx>
Reviewed-by: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
Documentation/memory-barriers.txt | 164 +++++++++++++++++++-------------------
1 file changed, 85 insertions(+), 79 deletions(-)
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -371,33 +371,38 @@ VARIETIES OF MEMORY BARRIER
And a couple of implicit varieties:
- (5) LOCK operations.
+ (5) ACQUIRE operations.
This acts as a one-way permeable barrier. It guarantees that all memory
- operations after the LOCK operation will appear to happen after the LOCK
- operation with respect to the other components of the system.
+ operations after the ACQUIRE operation will appear to happen after the
+ ACQUIRE operation with respect to the other components of the system.
+ ACQUIRE operations include LOCK operations and smp_load_acquire()
+ operations.
- Memory operations that occur before a LOCK operation may appear to happen
- after it completes.
+ Memory operations that occur before a ACQUIRE operation may appear to
+ happen after it completes.
- A LOCK operation should almost always be paired with an UNLOCK operation.
+ A ACQUIRE operation should almost always be paired with an RELEASE
+ operation.
- (6) UNLOCK operations.
+ (6) RELEASE operations.
This also acts as a one-way permeable barrier. It guarantees that all
- memory operations before the UNLOCK operation will appear to happen before
- the UNLOCK operation with respect to the other components of the system.
+ memory operations before the RELEASE operation will appear to happen
+ before the RELEASE operation with respect to the other components of the
+ system. RELEASE operations include UNLOCK operations and
+ smp_store_release() operations.
- Memory operations that occur after an UNLOCK operation may appear to
+ Memory operations that occur after an RELEASE operation may appear to
happen before it completes.
- LOCK and UNLOCK operations are guaranteed to appear with respect to each
- other strictly in the order specified.
+ ACQUIRE and RELEASE operations are guaranteed to appear with respect to
+ each other strictly in the order specified.
- The use of LOCK and UNLOCK operations generally precludes the need for
- other sorts of memory barrier (but note the exceptions mentioned in the
- subsection "MMIO write barrier").
+ The use of ACQUIRE and RELEASE operations generally precludes the need
+ for other sorts of memory barrier (but note the exceptions mentioned in
+ the subsection "MMIO write barrier").
Memory barriers are only required where there's a possibility of interaction
@@ -1135,7 +1140,7 @@ CPU from reordering them.
clear_bit( ... );
This prevents memory operations before the clear leaking to after it. See
- the subsection on "Locking Functions" with reference to UNLOCK operation
+ the subsection on "Locking Functions" with reference to RELEASE operation
implications.
See Documentation/atomic_ops.txt for more information. See the "Atomic
@@ -1181,65 +1186,66 @@ LOCKING FUNCTIONS
(*) R/W semaphores
(*) RCU
-In all cases there are variants on "LOCK" operations and "UNLOCK" operations
+In all cases there are variants on "ACQUIRE" operations and "RELEASE" operations
for each construct. These operations all imply certain barriers:
- (1) LOCK operation implication:
+ (1) ACQUIRE operation implication:
- Memory operations issued after the LOCK will be completed after the LOCK
- operation has completed.
+ Memory operations issued after the ACQUIRE will be completed after the
+ ACQUIRE operation has completed.
- Memory operations issued before the LOCK may be completed after the LOCK
- operation has completed.
+ Memory operations issued before the ACQUIRE may be completed after the
+ ACQUIRE operation has completed.
- (2) UNLOCK operation implication:
+ (2) RELEASE operation implication:
- Memory operations issued before the UNLOCK will be completed before the
- UNLOCK operation has completed.
+ Memory operations issued before the RELEASE will be completed before the
+ RELEASE operation has completed.
- Memory operations issued after the UNLOCK may be completed before the
- UNLOCK operation has completed.
+ Memory operations issued after the RELEASE may be completed before the
+ RELEASE operation has completed.
- (3) LOCK vs LOCK implication:
+ (3) ACQUIRE vs ACQUIRE implication:
- All LOCK operations issued before another LOCK operation will be completed
- before that LOCK operation.
+ All ACQUIRE operations issued before another ACQUIRE operation will be
+ completed before that ACQUIRE operation.
- (4) LOCK vs UNLOCK implication:
+ (4) ACQUIRE vs RELEASE implication:
- All LOCK operations issued before an UNLOCK operation will be completed
- before the UNLOCK operation.
+ All ACQUIRE operations issued before an RELEASE operation will be
+ completed before the RELEASE operation.
- All UNLOCK operations issued before a LOCK operation will be completed
- before the LOCK operation.
+ All RELEASE operations issued before a ACQUIRE operation will be
+ completed before the ACQUIRE operation.
- (5) Failed conditional LOCK implication:
+ (5) Failed conditional ACQUIRE implication:
- Certain variants of the LOCK operation may fail, either due to being
+ Certain variants of the ACQUIRE operation may fail, either due to being
unable to get the lock immediately, or due to receiving an unblocked
- signal whilst asleep waiting for the lock to become available. Failed
- locks do not imply any sort of barrier.
+ signal whilst asleep waiting for the lock to become available. For
+ example, failed locks do not imply any sort of barrier.
-Therefore, from (1), (2) and (4) an UNLOCK followed by an unconditional LOCK is
-equivalent to a full barrier, but a LOCK followed by an UNLOCK is not.
+Therefore, from (1), (2) and (4) an RELEASE followed by an unconditional
+ACQUIRE is equivalent to a full barrier, but a ACQUIRE followed by an RELEASE
+is not.
[!] Note: one of the consequences of LOCKs and UNLOCKs being only one-way
barriers is that the effects of instructions outside of a critical section
may seep into the inside of the critical section.
-A LOCK followed by an UNLOCK may not be assumed to be full memory barrier
-because it is possible for an access preceding the LOCK to happen after the
-LOCK, and an access following the UNLOCK to happen before the UNLOCK, and the
-two accesses can themselves then cross:
+A ACQUIRE followed by an RELEASE may not be assumed to be full memory barrier
+because it is possible for an access preceding the ACQUIRE to happen after the
+ACQUIRE, and an access following the RELEASE to happen before the RELEASE, and
+the two accesses can themselves then cross:
*A = a;
- LOCK
- UNLOCK
+ ACQUIRE
+ RELEASE
*B = b;
may occur as:
- LOCK, STORE *B, STORE *A, UNLOCK
+ ACQUIRE, STORE *B, STORE *A, RELEASE
Locks and semaphores may not provide any guarantee of ordering on UP compiled
systems, and so cannot be counted on in such a situation to actually achieve
@@ -1253,33 +1259,33 @@ See also the section on "Inter-CPU locki
*A = a;
*B = b;
- LOCK
+ ACQUIRE
*C = c;
*D = d;
- UNLOCK
+ RELEASE
*E = e;
*F = f;
The following sequence of events is acceptable:
- LOCK, {*F,*A}, *E, {*C,*D}, *B, UNLOCK
+ ACQUIRE, {*F,*A}, *E, {*C,*D}, *B, RELEASE
[+] Note that {*F,*A} indicates a combined access.
But none of the following are:
- {*F,*A}, *B, LOCK, *C, *D, UNLOCK, *E
- *A, *B, *C, LOCK, *D, UNLOCK, *E, *F
- *A, *B, LOCK, *C, UNLOCK, *D, *E, *F
- *B, LOCK, *C, *D, UNLOCK, {*F,*A}, *E
+ {*F,*A}, *B, ACQUIRE, *C, *D, RELEASE, *E
+ *A, *B, *C, ACQUIRE, *D, RELEASE, *E, *F
+ *A, *B, ACQUIRE, *C, RELEASE, *D, *E, *F
+ *B, ACQUIRE, *C, *D, RELEASE, {*F,*A}, *E
INTERRUPT DISABLING FUNCTIONS
-----------------------------
-Functions that disable interrupts (LOCK equivalent) and enable interrupts
-(UNLOCK equivalent) will act as compiler barriers only. So if memory or I/O
+Functions that disable interrupts (ACQUIRE equivalent) and enable interrupts
+(RELEASE equivalent) will act as compiler barriers only. So if memory or I/O
barriers are required in such a situation, they must be provided from some
other means.
@@ -1436,24 +1442,24 @@ Consider the following: the system has a
CPU 1 CPU 2
=============================== ===============================
*A = a; *E = e;
- LOCK M LOCK Q
+ ACQUIRE M ACQUIRE Q
*B = b; *F = f;
*C = c; *G = g;
- UNLOCK M UNLOCK Q
+ RELEASE M RELEASE Q
*D = d; *H = h;
Then there is no guarantee as to what order CPU 3 will see the accesses to *A
through *H occur in, other than the constraints imposed by the separate locks
on the separate CPUs. It might, for example, see:
- *E, LOCK M, LOCK Q, *G, *C, *F, *A, *B, UNLOCK Q, *D, *H, UNLOCK M
+ *E, ACQUIRE M, ACQUIRE Q, *G, *C, *F, *A, *B, RELEASE Q, *D, *H, RELEASE M
But it won't see any of:
- *B, *C or *D preceding LOCK M
- *A, *B or *C following UNLOCK M
- *F, *G or *H preceding LOCK Q
- *E, *F or *G following UNLOCK Q
+ *B, *C or *D preceding ACQUIRE M
+ *A, *B or *C following RELEASE M
+ *F, *G or *H preceding ACQUIRE Q
+ *E, *F or *G following RELEASE Q
However, if the following occurs:
@@ -1461,28 +1467,28 @@ through *H occur in, other than the cons
CPU 1 CPU 2
=============================== ===============================
*A = a;
- LOCK M [1]
+ ACQUIRE M [1]
*B = b;
*C = c;
- UNLOCK M [1]
+ RELEASE M [1]
*D = d; *E = e;
- LOCK M [2]
+ ACQUIRE M [2]
*F = f;
*G = g;
- UNLOCK M [2]
+ RELEASE M [2]
*H = h;
CPU 3 might see:
- *E, LOCK M [1], *C, *B, *A, UNLOCK M [1],
- LOCK M [2], *H, *F, *G, UNLOCK M [2], *D
+ *E, ACQUIRE M [1], *C, *B, *A, RELEASE M [1],
+ ACQUIRE M [2], *H, *F, *G, RELEASE M [2], *D
But assuming CPU 1 gets the lock first, CPU 3 won't see any of:
- *B, *C, *D, *F, *G or *H preceding LOCK M [1]
- *A, *B or *C following UNLOCK M [1]
- *F, *G or *H preceding LOCK M [2]
- *A, *B, *C, *E, *F or *G following UNLOCK M [2]
+ *B, *C, *D, *F, *G or *H preceding ACQUIRE M [1]
+ *A, *B or *C following RELEASE M [1]
+ *F, *G or *H preceding ACQUIRE M [2]
+ *A, *B, *C, *E, *F or *G following RELEASE M [2]
LOCKS VS I/O ACCESSES
@@ -1702,13 +1708,13 @@ about the state (old or new) implies an
test_and_clear_bit();
test_and_change_bit();
-These are used for such things as implementing LOCK-class and UNLOCK-class
+These are used for such things as implementing ACQUIRE-class and RELEASE-class
operations and adjusting reference counters towards object destruction, and as
such the implicit memory barrier effects are necessary.
The following operations are potential problems as they do _not_ imply memory
-barriers, but might be used for implementing such things as UNLOCK-class
+barriers, but might be used for implementing such things as RELEASE-class
operations:
atomic_set();
@@ -1750,9 +1756,9 @@ barriers are needed or not.
clear_bit_unlock();
__clear_bit_unlock();
-These implement LOCK-class and UNLOCK-class operations. These should be used in
-preference to other operations when implementing locking primitives, because
-their implementations can be optimised on many architectures.
+These implement ACQUIRE-class and RELEASE-class operations. These should be
+used in preference to other operations when implementing locking primitives,
+because their implementations can be optimised on many architectures.
[!] Note that special memory barrier primitives are available for these
situations because on some CPUs the atomic instructions used imply full memory
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