More than one kernel developer has expressed the opinion that the LKMM
should enforce ordering of writes by locking. In other words, given
the following code:
WRITE_ONCE(x, 1);
spin_unlock(&s):
spin_lock(&s);
WRITE_ONCE(y, 1);
the stores to x and y should be propagated in order to all other CPUs,
even though those other CPUs might not access the lock s. In terms of
the memory model, this means expanding the cumul-fence relation.
Locks should also provide read-read (and read-write) ordering in a
similar way. Given:
READ_ONCE(x);
spin_unlock(&s);
spin_lock(&s);
READ_ONCE(y); // or WRITE_ONCE(y, 1);
the load of x should be executed before the load of (or store to) y.
The LKMM already provides this ordering, but it provides it even in
the case where the two accesses are separated by a release/acquire
pair of fences rather than unlock/lock. This would prevent
architectures from using weakly ordered implementations of release and
acquire, which seems like an unnecessary restriction. The patch
therefore removes the ordering requirement from the LKMM for that
case.
There are several arguments both for and against this change. Let us
refer to these enhanced ordering properties by saying that the LKMM
would require locks to be RCtso (a bit of a misnomer, but analogous to
RCpc and RCsc) and it would require ordinary acquire/release only to
be RCpc. (Note: In the following, the phrase "all supported
architectures" is meant not to include RISC-V. Although RISC-V is
indeed supported by the kernel, the implementation is still somewhat
in a state of flux and therefore statements about it would be
premature.)
Pros:
The kernel already provides RCtso ordering for locks on all
supported architectures, even though this is not stated
explicitly anywhere. Therefore the LKMM should formalize it.
In theory, guaranteeing RCtso ordering would reduce the need
for additional barrier-like constructs meant to increase the
ordering strength of locks.
Will Deacon and Peter Zijlstra are strongly in favor of
formalizing the RCtso requirement. Linus Torvalds and Will
would like to go even further, requiring locks to have RCsc
behavior (ordering preceding writes against later reads), but
they recognize that this would incur a noticeable performance
degradation on the POWER architecture. Linus also points out
that people have made the mistake, in the past, of assuming
that locking has stronger ordering properties than is
currently guaranteed, and this change would reduce the
likelihood of such mistakes.
Not requiring ordinary acquire/release to be any stronger than
RCpc may prove advantageous for future architectures, allowing
them to implement smp_load_acquire() and smp_store_release()
with more efficient machine instructions than would be
possible if the operations had to be RCtso. Will and Linus
approve this rationale, hypothetical though it is at the
moment (it may end up affecting the RISC-V implementation).
The same argument may or may not apply to RMW-acquire/release;
see also the second Con entry below.
Linus feels that locks should be easy for people to use
without worrying about memory consistency issues, since they
are so pervasive in the kernel, whereas acquire/release is
much more of an "experts only" tool. Requiring locks to be
RCtso is a step in this direction.
Cons:
Andrea Parri and Luc Maranget think that locks should have the
same ordering properties as ordinary acquire/release (indeed,
Luc points out that the names "acquire" and "release" derive
from the usage of locks). Andrea points out that having
different ordering properties for different forms of acquires
and releases is not only unnecessary, it would also be
confusing and unmaintainable.
Locks are constructed from lower-level primitives, typically
RMW-acquire (for locking) and ordinary release (for unlock).
It is illogical to require stronger ordering properties from
the high-level operations than from the low-level operations
they comprise. Thus, this change would make
while (cmpxchg_acquire(&s, 0, 1) != 0)
cpu_relax();
an incorrect implementation of spin_lock(&s) as far as the
LKMM is concerned. In theory this weakness can be ameliorated
by changing the LKMM even further, requiring
RMW-acquire/release also to be RCtso (which it already is on
all supported architectures).
As far as I know, nobody has singled out any examples of code
in the kernel that actually relies on locks being RCtso.
(People mumble about RCU and the scheduler, but nobody has
pointed to any actual code. If there are any real cases,
their number is likely quite small.) If RCtso ordering is not
needed, why require it?
A handful of locking constructs (qspinlocks, qrwlocks, and
mcs_spinlocks) are built on top of smp_cond_load_acquire()
instead of an RMW-acquire instruction. It currently provides
only the ordinary acquire semantics, not the stronger ordering
this patch would require of locks. In theory this could be
ameliorated by requiring smp_cond_load_acquire() in
combination with ordinary release also to be RCtso (which is
currently true on all supported architectures).
On future weakly ordered architectures, people may be able to
implement locks in a non-RCtso fashion with significant
performance improvement. Meeting the RCtso requirement would
necessarily add run-time overhead.
Overall, the technical aspects of these arguments seem relatively
minor, and it appears mostly to boil down to a matter of opinion.
Since the opinions of senior kernel maintainers such as Linus,
Peter, and Will carry more weight than those of Luc and Andrea, this
patch changes the model in accordance with the maintainers' wishes.
Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Will Deacon <will.deacon@xxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>
---
v.5: Incorporated feedback from Andrea regarding the updated Changelog.
v.4: Added pros and cons discussion to the Changelog.
v.3: Rebased against the dev branch of Paul's linux-rcu tree.
Changed unlock-rf-lock-po to po-unlock-rf-lock-po, making it more
symmetrical and more in accordance with the use of fence.tso for
the release on RISC-V.
v.2: Restrict the ordering to lock operations, not general release
and acquire fences.
[as1871e]
tools/memory-model/Documentation/explanation.txt | 186 +++++++---
tools/memory-model/linux-kernel.cat | 8
tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus | 7
3 files changed, 150 insertions(+), 51 deletions(-)
Index: usb-4.x/tools/memory-model/linux-kernel.cat
===================================================================
--- usb-4.x.orig/tools/memory-model/linux-kernel.cat
+++ usb-4.x/tools/memory-model/linux-kernel.cat
@@ -38,7 +38,7 @@ let strong-fence = mb | gp
(* Release Acquire *)
let acq-po = [Acquire] ; po ; [M]
let po-rel = [M] ; po ; [Release]
-let rfi-rel-acq = [Release] ; rfi ; [Acquire]
+let po-unlock-rf-lock-po = po ; [UL] ; rf ; [LKR] ; po
(**********************************)
(* Fundamental coherence ordering *)
@@ -60,13 +60,13 @@ let dep = addr | data
let rwdep = (dep | ctrl) ; [W]
let overwrite = co | fr
let to-w = rwdep | (overwrite & int)
-let to-r = addr | (dep ; rfi) | rfi-rel-acq
+let to-r = addr | (dep ; rfi)
let fence = strong-fence | wmb | po-rel | rmb | acq-po
-let ppo = to-r | to-w | fence
+let ppo = to-r | to-w | fence | (po-unlock-rf-lock-po & int)
(* Propagation: Ordering from release operations and strong fences. *)
let A-cumul(r) = rfe? ; r
-let cumul-fence = A-cumul(strong-fence | po-rel) | wmb
+let cumul-fence = A-cumul(strong-fence | po-rel) | wmb | po-unlock-rf-lock-po
let prop = (overwrite & ext)? ; cumul-fence* ; rfe?
(*
Index: usb-4.x/tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus
===================================================================
--- usb-4.x.orig/tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus
+++ usb-4.x/tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus
@@ -1,11 +1,10 @@
C ISA2+pooncelock+pooncelock+pombonce
(*
- * Result: Sometimes
+ * Result: Never
*
- * This test shows that the ordering provided by a lock-protected S
- * litmus test (P0() and P1()) are not visible to external process P2().
- * This is likely to change soon.
+ * This test shows that write-write ordering provided by locks
+ * (in P0() and P1()) is visible to external process P2().
*)
{}
Index: usb-4.x/tools/memory-model/Documentation/explanation.txt
===================================================================
--- usb-4.x.orig/tools/memory-model/Documentation/explanation.txt
+++ usb-4.x/tools/memory-model/Documentation/explanation.txt
@@ -28,7 +28,8 @@ Explanation of the Linux-Kernel Memory C
20. THE HAPPENS-BEFORE RELATION: hb
21. THE PROPAGATES-BEFORE RELATION: pb
22. RCU RELATIONS: rcu-link, gp, rscs, rcu-fence, and rb
- 23. ODDS AND ENDS
+ 23. LOCKING
+ 24. ODDS AND ENDS
@@ -1067,28 +1068,6 @@ allowing out-of-order writes like this t
violating the write-write coherence rule by requiring the CPU not to
send the W write to the memory subsystem at all!)
-There is one last example of preserved program order in the LKMM: when
-a load-acquire reads from an earlier store-release. For example:
-
- smp_store_release(&x, 123);
- r1 = smp_load_acquire(&x);
-
-If the smp_load_acquire() ends up obtaining the 123 value that was
-stored by the smp_store_release(), the LKMM says that the load must be
-executed after the store; the store cannot be forwarded to the load.
-This requirement does not arise from the operational model, but it
-yields correct predictions on all architectures supported by the Linux
-kernel, although for differing reasons.
-
-On some architectures, including x86 and ARMv8, it is true that the
-store cannot be forwarded to the load. On others, including PowerPC
-and ARMv7, smp_store_release() generates object code that starts with
-a fence and smp_load_acquire() generates object code that ends with a
-fence. The upshot is that even though the store may be forwarded to
-the load, it is still true that any instruction preceding the store
-will be executed before the load or any following instructions, and
-the store will be executed before any instruction following the load.
-
AND THEN THERE WAS ALPHA
------------------------
@@ -1766,6 +1745,147 @@ before it does, and the critical section
grace period does and ends after it does.
+LOCKING
+-------
+
+The LKMM includes locking. In fact, there is special code for locking
+in the formal model, added in order to make tools run faster.
+However, this special code is intended to be more or less equivalent
+to concepts we have already covered. A spinlock_t variable is treated
+the same as an int, and spin_lock(&s) is treated almost the same as:
+
+ while (cmpxchg_acquire(&s, 0, 1) != 0)
+ cpu_relax();
+
+This waits until s is equal to 0 and then atomically sets it to 1,
+and the read part of the cmpxchg operation acts as an acquire fence.
+An alternate way to express the same thing would be:
+
+ r = xchg_acquire(&s, 1);
+
+along with a requirement that at the end, r = 0. Similarly,
+spin_trylock(&s) is treated almost the same as:
+
+ return !cmpxchg_acquire(&s, 0, 1);
+
+which atomically sets s to 1 if it is currently equal to 0 and returns
+true if it succeeds (the read part of the cmpxchg operation acts as an
+acquire fence only if the operation is successful). spin_unlock(&s)
+is treated almost the same as:
+
+ smp_store_release(&s, 0);
+
+The "almost" qualifiers above need some explanation. In the LKMM, the
+store-release in a spin_unlock() and the load-acquire which forms the
+first half of the atomic rmw update in a spin_lock() or a successful
+spin_trylock() -- we can call these things lock-releases and
+lock-acquires -- have two properties beyond those of ordinary releases
+and acquires.
+
+First, when a lock-acquire reads from a lock-release, the LKMM
+requires that every instruction po-before the lock-release must
+execute before any instruction po-after the lock-acquire. This would
+naturally hold if the release and acquire operations were on different
+CPUs, but the LKMM says it holds even when they are on the same CPU.
+For example:
+
+ int x, y;
+ spinlock_t s;
+
+ P0()
+ {
+ int r1, r2;
+
+ spin_lock(&s);
+ r1 = READ_ONCE(x);
+ spin_unlock(&s);
+ spin_lock(&s);
+ r2 = READ_ONCE(y);
+ spin_unlock(&s);
+ }
+
+ P1()
+ {
+ WRITE_ONCE(y, 1);
+ smp_wmb();
+ WRITE_ONCE(x, 1);
+ }
+
+Here the second spin_lock() reads from the first spin_unlock(), and
+therefore the load of x must execute before the load of y. Thus we
+cannot have r1 = 1 and r2 = 0 at the end (this is an instance of the
+MP pattern).
+
+This requirement does not apply to ordinary release and acquire
+fences, only to lock-related operations. For instance, suppose P0()
+in the example had been written as:
+
+ P0()
+ {
+ int r1, r2, r3;
+
+ r1 = READ_ONCE(x);
+ smp_store_release(&s, 1);
+ r3 = smp_load_acquire(&s);
+ r2 = READ_ONCE(y);
+ }
+
+Then the CPU would be allowed to forward the s = 1 value from the
+smp_store_release() to the smp_load_acquire(), executing the
+instructions in the following order:
+
+ r3 = smp_load_acquire(&s); // Obtains r3 = 1
+ r2 = READ_ONCE(y);
+ r1 = READ_ONCE(x);
+ smp_store_release(&s, 1); // Value is forwarded
+
+and thus it could load y before x, obtaining r2 = 0 and r1 = 1.
+
+Second, when a lock-acquire reads from a lock-release, and some other
+stores W and W' occur po-before the lock-release and po-after the
+lock-acquire respectively, the LKMM requires that W must propagate to
+each CPU before W' does. For example, consider:
+
+ int x, y;
+ spinlock_t x;
+
+ P0()
+ {
+ spin_lock(&s);
+ WRITE_ONCE(x, 1);
+ spin_unlock(&s);
+ }
+
+ P1()
+ {
+ int r1;
+
+ spin_lock(&s);
+ r1 = READ_ONCE(x);
+ WRITE_ONCE(y, 1);
+ spin_unlock(&s);
+ }
+
+ P2()
+ {
+ int r2, r3;
+
+ r2 = READ_ONCE(y);
+ smp_rmb();
+ r3 = READ_ONCE(x);
+ }
+
+If r1 = 1 at the end then the spin_lock() in P1 must have read from
+the spin_unlock() in P0. Hence the store to x must propagate to P2
+before the store to y does, so we cannot have r2 = 1 and r3 = 0.
+
+These two special requirements for lock-release and lock-acquire do
+not arise from the operational model. Nevertheless, kernel developers
+have come to expect and rely on them because they do hold on all
+architectures supported by the Linux kernel, albeit for various
+differing reasons.
+
+
ODDS AND ENDS
-------------
@@ -1831,26 +1951,6 @@ they behave as follows:
events and the events preceding them against all po-later
events.
-The LKMM includes locking. In fact, there is special code for locking
-in the formal model, added in order to make tools run faster.
-However, this special code is intended to be exactly equivalent to
-concepts we have already covered. A spinlock_t variable is treated
-the same as an int, and spin_lock(&s) is treated the same as:
-
- while (cmpxchg_acquire(&s, 0, 1) != 0)
- cpu_relax();
-
-which waits until s is equal to 0 and then atomically sets it to 1,
-and where the read part of the atomic update is also an acquire fence.
-An alternate way to express the same thing would be:
-
- r = xchg_acquire(&s, 1);
-
-along with a requirement that at the end, r = 0. spin_unlock(&s) is
-treated the same as:
-
- smp_store_release(&s, 0);
-
Interestingly, RCU and locking each introduce the possibility of
deadlock. When faced with code sequences such as:
