The attached patch applies some alterations to the memory barrier document that
I worked out with Paul McKenney of IBM, plus some of the alterations suggested
by Alan Stern.
The following changes were made:
(*) One of the examples given for what can happen with overlapping memory
barriers was wrong.
(*) The description of general memory barriers said that a general barrier is
a combination of a read barrier and a write barrier. This isn't entirely
true: it implies both, but is more than a combination of both.
(*) The first example in the "SMP Barrier Pairing" section was wrong: the
loads around the read barrier need to touch the memory locations in the
opposite order to the stores around the write barrier.
(*) Added a note to make explicit that the loads should be in reverse order to
the stores.
(*) Adjusted the diagrams in the "Examples Of Memory Barrier Sequences"
section to make them clearer. Added a couple of diagrams to make it more
clear as to how it could go wrong without the barrier.
(*) Added a section on memory speculation.
(*) Dropped any references to memory allocation routines doing memory
barriers. They may do sometimes, but it can't be relied on. This may be
worthy of further documentation later.
(*) Made the fact that a LOCK followed by an UNLOCK should not be considered a
full memory barrier more explicit and gave an example.
Signed-Off-By: David Howells <[email protected]>
---
warthog>diffstat -p1 /tmp/mb.diff
Documentation/memory-barriers.txt | 348 +++++++++++++++++++++++++++++---------
1 file changed, 270 insertions(+), 78 deletions(-)
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index c61d8b8..4710845 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -19,6 +19,7 @@ Contents:
- Control dependencies.
- SMP barrier pairing.
- Examples of memory barrier sequences.
+ - Read memory barriers vs load speculation.
(*) Explicit kernel barriers.
@@ -248,7 +249,7 @@ And there are a number of things that _m
we may get either of:
STORE *A = X; Y = LOAD *A;
- STORE *A = Y;
+ STORE *A = Y = X;
=========================
@@ -344,9 +345,12 @@ Memory barriers come in four basic varie
(4) General memory barriers.
- A general memory barrier is a combination of both a read memory barrier
- and a write memory barrier. It is a partial ordering over both loads and
- stores.
+ A general memory barrier gives a guarantee that all the LOAD and STORE
+ operations specified before the barrier will appear to happen before all
+ the LOAD and STORE operations specified after the barrier with respect to
+ the other components of the system.
+
+ A general memory barrier is a partial ordering over both loads and stores.
General memory barriers imply both read and write memory barriers, and so
can substitute for either.
@@ -546,9 +550,9 @@ write barrier, though, again, a general
=============== ===============
a = 1;
<write barrier>
- b = 2; x = a;
+ b = 2; x = b;
<read barrier>
- y = b;
+ y = a;
Or:
@@ -563,6 +567,18 @@ Or:
Basically, the read barrier always has to be there, even though it can be of
the "weaker" type.
+[!] Note that the stores before the write barrier would normally be expected to
+match the loads after the read barrier or data dependency barrier, and vice
+versa:
+
+ CPU 1 CPU 2
+ =============== ===============
+ a = 1; }---- --->{ v = c
+ b = 2; } \ / { w = d
+ <write barrier> \ <read barrier>
+ c = 3; } / \ { x = a;
+ d = 4; }---- --->{ y = b;
+
EXAMPLES OF MEMORY BARRIER SEQUENCES
------------------------------------
@@ -600,8 +616,8 @@ STORE B, STORE C } all occuring before t
| | +------+
+-------+ : :
|
- | Sequence in which stores committed to memory system
- | by CPU 1
+ | Sequence in which stores are committed to the
+ | memory system by CPU 1
V
@@ -683,14 +699,12 @@ then the following will occur:
| : : | |
| : : | CPU 2 |
| +-------+ | |
- \ | X->9 |------>| |
- \ +-------+ | |
- ----->| B->2 | | |
- +-------+ | |
- Makes sure all effects ---> ddddddddddddddddd | |
- prior to the store of C +-------+ | |
- are perceptible to | B->2 |------>| |
- successive loads +-------+ | |
+ | | X->9 |------>| |
+ | +-------+ | |
+ Makes sure all effects ---> \ ddddddddddddddddd | |
+ prior to the store of C \ +-------+ | |
+ are perceptible to ----->| B->2 |------>| |
+ subsequent loads +-------+ | |
: : +-------+
@@ -699,73 +713,239 @@ following sequence of events:
CPU 1 CPU 2
======================= =======================
+ { A = 0, B = 9 }
STORE A=1
- STORE B=2
- STORE C=3
<write barrier>
- STORE D=4
- STORE E=5
- LOAD A
+ STORE B=2
LOAD B
- LOAD C
- LOAD D
- LOAD E
+ LOAD A
Without intervention, CPU 2 may then choose to perceive the events on CPU 1 in
some effectively random order, despite the write barrier issued by CPU 1:
- +-------+ : :
- | | +------+
- | |------>| C=3 | }
- | | : +------+ }
- | | : | A=1 | }
- | | : +------+ }
- | CPU 1 | : | B=2 | }---
- | | +------+ } \
- | | wwwwwwwwwwwww} \
- | | +------+ } \ : : +-------+
- | | : | E=5 | } \ +-------+ | |
- | | : +------+ } \ { | C->3 |------>| |
- | |------>| D=4 | } \ { +-------+ : | |
- | | +------+ \ { | E->5 | : | |
- +-------+ : : \ { +-------+ : | |
- Transfer -->{ | A->1 | : | CPU 2 |
- from CPU 1 { +-------+ : | |
- to CPU 2 { | D->4 | : | |
- { +-------+ : | |
- { | B->2 |------>| |
- +-------+ | |
- : : +-------+
-
-
-If, however, a read barrier were to be placed between the load of C and the
-load of D on CPU 2, then the partial ordering imposed by CPU 1 will be
-perceived correctly by CPU 2.
+ +-------+ : : : :
+ | | +------+ +-------+
+ | |------>| A=1 |------ --->| A->0 |
+ | | +------+ \ +-------+
+ | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
+ | | +------+ | +-------+
+ | |------>| B=2 |--- | : :
+ | | +------+ \ | : : +-------+
+ +-------+ : : \ | +-------+ | |
+ ---------->| B->2 |------>| |
+ | +-------+ | CPU 2 |
+ | | A->0 |------>| |
+ | +-------+ | |
+ | : : +-------+
+ \ : :
+ \ +-------+
+ ---->| A->1 |
+ +-------+
+ : :
- +-------+ : :
- | | +------+
- | |------>| C=3 | }
- | | : +------+ }
- | | : | A=1 | }---
- | | : +------+ } \
- | CPU 1 | : | B=2 | } \
- | | +------+ \
- | | wwwwwwwwwwwwwwww \
- | | +------+ \ : : +-------+
- | | : | E=5 | } \ +-------+ | |
- | | : +------+ }--- \ { | C->3 |------>| |
- | |------>| D=4 | } \ \ { +-------+ : | |
- | | +------+ \ -->{ | B->2 | : | |
- +-------+ : : \ { +-------+ : | |
- \ { | A->1 | : | CPU 2 |
- \ +-------+ | |
- At this point the read ----> \ rrrrrrrrrrrrrrrrr | |
- barrier causes all effects \ +-------+ | |
- prior to the storage of C \ { | E->5 | : | |
- to be perceptible to CPU 2 -->{ +-------+ : | |
- { | D->4 |------>| |
- +-------+ | |
- : : +-------+
+
+If, however, a read barrier were to be placed between the load of E and the
+load of A on CPU 2:
+
+ CPU 1 CPU 2
+ ======================= =======================
+ { A = 0, B = 9 }
+ STORE A=1
+ <write barrier>
+ STORE B=2
+ LOAD B
+ <read barrier>
+ LOAD A
+
+then the partial ordering imposed by CPU 1 will be perceived correctly by CPU
+2:
+
+ +-------+ : : : :
+ | | +------+ +-------+
+ | |------>| A=1 |------ --->| A->0 |
+ | | +------+ \ +-------+
+ | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
+ | | +------+ | +-------+
+ | |------>| B=2 |--- | : :
+ | | +------+ \ | : : +-------+
+ +-------+ : : \ | +-------+ | |
+ ---------->| B->2 |------>| |
+ | +-------+ | CPU 2 |
+ | : : | |
+ | : : | |
+ At this point the read ----> \ rrrrrrrrrrrrrrrrr | |
+ barrier causes all effects \ +-------+ | |
+ prior to the storage of B ---->| A->1 |------>| |
+ to be perceptible to CPU 2 +-------+ | |
+ : : +-------+
+
+
+To illustrate this more completely, consider what could happen if the code
+contained a load of A either side of the read barrier:
+
+ CPU 1 CPU 2
+ ======================= =======================
+ { A = 0, B = 9 }
+ STORE A=1
+ <write barrier>
+ STORE B=2
+ LOAD B
+ LOAD A [first load of A]
+ <read barrier>
+ LOAD A [second load of A]
+
+Even though the two loads of A both occur after the load of B, they may both
+come up with different values:
+
+ +-------+ : : : :
+ | | +------+ +-------+
+ | |------>| A=1 |------ --->| A->0 |
+ | | +------+ \ +-------+
+ | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
+ | | +------+ | +-------+
+ | |------>| B=2 |--- | : :
+ | | +------+ \ | : : +-------+
+ +-------+ : : \ | +-------+ | |
+ ---------->| B->2 |------>| |
+ | +-------+ | CPU 2 |
+ | : : | |
+ | : : | |
+ | +-------+ | |
+ | | A->0 |------>| 1st |
+ | +-------+ | |
+ At this point the read ----> \ rrrrrrrrrrrrrrrrr | |
+ barrier causes all effects \ +-------+ | |
+ prior to the storage of B ---->| A->1 |------>| 2nd |
+ to be perceptible to CPU 2 +-------+ | |
+ : : +-------+
+
+
+But it may be that the update to A from CPU 1 becomes perceptible to CPU 2
+before the read barrier completes anyway:
+
+ +-------+ : : : :
+ | | +------+ +-------+
+ | |------>| A=1 |------ --->| A->0 |
+ | | +------+ \ +-------+
+ | CPU 1 | wwwwwwwwwwwwwwww \ --->| B->9 |
+ | | +------+ | +-------+
+ | |------>| B=2 |--- | : :
+ | | +------+ \ | : : +-------+
+ +-------+ : : \ | +-------+ | |
+ ---------->| B->2 |------>| |
+ | +-------+ | CPU 2 |
+ | : : | |
+ \ : : | |
+ \ +-------+ | |
+ ---->| A->1 |------>| 1st |
+ +-------+ | |
+ rrrrrrrrrrrrrrrrr | |
+ +-------+ | |
+ | A->1 |------>| 2nd |
+ +-------+ | |
+ : : +-------+
+
+
+The guarantee is that the second load will always come up with A == 1 if the
+load of B came up with B == 2. No such guarantee exists for the first load of
+A; that may come up with either A == 0 or A == 1.
+
+
+READ MEMORY BARRIERS VS LOAD SPECULATION
+----------------------------------------
+
+Many CPUs speculate with loads: that is they see that they will need to load an
+item from memory, and they find a time where they're not using the bus for any
+other loads, and so do the load in advance - even though they haven't actually
+got to that point in the instruction execution flow yet. This permits the
+actual load instruction to potentially complete immediately because the CPU
+already has the value to hand.
+
+It may turn out that the CPU didn't actually need the value - perhaps because a
+branch circumvented the load - in which case it can discard the value or just
+cache it for later use.
+
+Consider:
+
+ CPU 1 CPU 2
+ ======================= =======================
+ LOAD B
+ DIVIDE } Divide instructions generally
+ DIVIDE } take a long time to perform
+ LOAD A
+
+Which might appear as this:
+
+ : : +-------+
+ +-------+ | |
+ --->| B->2 |------>| |
+ +-------+ | CPU 2 |
+ : :DIVIDE | |
+ +-------+ | |
+ The CPU being busy doing a ---> --->| A->0 |~~~~ | |
+ division speculates on the +-------+ ~ | |
+ LOAD of A : : ~ | |
+ : :DIVIDE | |
+ : : ~ | |
+ Once the divisions are complete --> : : ~-->| |
+ the CPU can then perform the : : | |
+ LOAD with immediate effect : : +-------+
+
+
+Placing a read barrier or a data dependency barrier just before the second
+load:
+
+ CPU 1 CPU 2
+ ======================= =======================
+ LOAD B
+ DIVIDE
+ DIVIDE
+ <read barrier>
+ LOAD A
+
+will force any value speculatively obtained to be reconsidered to an extent
+dependent on the type of barrier used. If there was no change made to the
+speculated memory location, then the speculated value will just be used:
+
+ : : +-------+
+ +-------+ | |
+ --->| B->2 |------>| |
+ +-------+ | CPU 2 |
+ : :DIVIDE | |
+ +-------+ | |
+ The CPU being busy doing a ---> --->| A->0 |~~~~ | |
+ division speculates on the +-------+ ~ | |
+ LOAD of A : : ~ | |
+ : :DIVIDE | |
+ : : ~ | |
+ : : ~ | |
+ rrrrrrrrrrrrrrrr~ | |
+ : : ~ | |
+ : : ~-->| |
+ : : | |
+ : : +-------+
+
+
+but if there was an update or an invalidation from another CPU pending, then
+the speculation will be cancelled and the value reloaded:
+
+ : : +-------+
+ +-------+ | |
+ --->| B->2 |------>| |
+ +-------+ | CPU 2 |
+ : :DIVIDE | |
+ +-------+ | |
+ The CPU being busy doing a ---> --->| A->0 |~~~~ | |
+ division speculates on the +-------+ ~ | |
+ LOAD of A : : ~ | |
+ : :DIVIDE | |
+ : : ~ | |
+ : : ~ | |
+ rrrrrrrrrrrrrrrrr | |
+ +-------+ | |
+ The speculation is discarded ---> --->| A->1 |------>| |
+ and an updated value is +-------+ | |
+ retrieved : : +-------+
========================
@@ -901,7 +1081,7 @@ IMPLICIT KERNEL MEMORY BARRIERS
===============================
Some of the other functions in the linux kernel imply memory barriers, amongst
-which are locking, scheduling and memory allocation functions.
+which are locking and scheduling functions.
This specification is a _minimum_ guarantee; any particular architecture may
provide more substantial guarantees, but these may not be relied upon outside
@@ -966,6 +1146,20 @@ equivalent to a full barrier, but a LOCK
barriers is that the effects 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 = a;
+ LOCK
+ UNLOCK
+ *B = b;
+
+may occur as:
+
+ LOCK, STORE *B, STORE *A, UNLOCK
+
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
anything at all - especially with respect to I/O accesses - unless combined
@@ -1016,8 +1210,6 @@ Other functions that imply barriers:
(*) schedule() and similar imply full memory barriers.
- (*) Memory allocation and release functions imply full memory barriers.
-
=================================
INTER-CPU LOCKING BARRIER EFFECTS
-
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