* Re: [PATCH] tools/memory-model: Provide extra ordering for unlock+lock pair on the same CPU
[not found] <20210930130823.2103688-1-boqun.feng@gmail.com>
@ 2021-10-01 1:19 ` Boqun Feng
2021-10-08 5:30 ` Michael Ellerman
0 siblings, 1 reply; 5+ messages in thread
From: Boqun Feng @ 2021-10-01 1:19 UTC (permalink / raw)
To: Linux Kernel Mailing List
Cc: Paul E . McKenney , Dan Lustig, Will Deacon, Peter Zijlstra,
Linus Torvalds, Alexander Shishkin, Peter Anvin, Andrea Parri,
Ingo Molnar, Vince Weaver, Thomas Gleixner, Jiri Olsa,
Arnaldo Carvalho de Melo, Stephane Eranian, palmer, paul.walmsley,
mpe, Alan Stern, linux-arch
(Add linux-arch in Cc list)
Architecture maintainers, this patch is about strengthening our memory
model a little bit, your inputs (confirmation, ack/nack, etc.) are
appreciated.
Regards,
Boqun
On Thu, Sep 30, 2021 at 09:08:23PM +0800, Boqun Feng wrote:
> A recent discussion[1] shows that we are in favor of strengthening the
> ordering of unlock + lock on the same CPU: a unlock and a po-after lock
> should provide the so-called RCtso ordering, that is a memory access S
> po-before the unlock should be ordered against a memory access R
> po-after the lock, unless S is a store and R is a load.
>
> The strengthening meets programmers' expection that "sequence of two
> locked regions to be ordered wrt each other" (from Linus), and can
> reduce the mental burden when using locks. Therefore add it in LKMM.
>
> [1]: https://lore.kernel.org/lkml/20210909185937.GA12379@rowland.harvard.edu/
>
> Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> Alan,
>
> I added the "Co-developed-by" and "Signed-off-by" tags since most of the
> work is done by you. Feel free to let me know if you want to change
> anything.
>
> Regards,
> Boqun
>
>
> .../Documentation/explanation.txt | 44 +++++++++++--------
> tools/memory-model/linux-kernel.cat | 6 +--
> ...LB+unlocklockonceonce+poacquireonce.litmus | 33 ++++++++++++++
> ...unlocklockonceonce+fencermbonceonce.litmus | 33 ++++++++++++++
> tools/memory-model/litmus-tests/README | 8 ++++
> 5 files changed, 102 insertions(+), 22 deletions(-)
> create mode 100644 tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus
> create mode 100644 tools/memory-model/litmus-tests/MP+unlocklockonceonce+fencermbonceonce.litmus
>
> diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
> index 5d72f3112e56..394ee57d58f2 100644
> --- a/tools/memory-model/Documentation/explanation.txt
> +++ b/tools/memory-model/Documentation/explanation.txt
> @@ -1813,15 +1813,16 @@ 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:
> +First, when a lock-acquire reads from or is po-after 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 and accessed the same lock variable, but the LKMM says
> +it also holds when they are on the same CPU, even if they access
> +different lock variables. For example:
>
> int x, y;
> - spinlock_t s;
> + spinlock_t s, t;
>
> P0()
> {
> @@ -1830,9 +1831,9 @@ For example:
> spin_lock(&s);
> r1 = READ_ONCE(x);
> spin_unlock(&s);
> - spin_lock(&s);
> + spin_lock(&t);
> r2 = READ_ONCE(y);
> - spin_unlock(&s);
> + spin_unlock(&t);
> }
>
> P1()
> @@ -1842,10 +1843,10 @@ For example:
> 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).
> +Here the second spin_lock() is po-after the first spin_unlock(), and
> +therefore the load of x must execute before the load of y, even though
> +the two locking operations use different locks. 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()
> @@ -1872,13 +1873,13 @@ instructions in the following order:
>
> 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:
> +Second, when a lock-acquire reads from or is po-after 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;
> + spinlock_t s;
>
> P0()
> {
> @@ -1908,7 +1909,12 @@ each CPU before W' does. For example, consider:
>
> 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.
> +before the store to y does, so we cannot have r2 = 1 and r3 = 0. But
> +if P1 had used a lock variable different from s, the writes could have
> +propagated in either order. (On the other hand, if the code in P0 and
> +P1 had all executed on a single CPU, as in the example before this
> +one, then the writes would have propagated in order even if the two
> +critical sections used different lock variables.)
>
> These two special requirements for lock-release and lock-acquire do
> not arise from the operational model. Nevertheless, kernel developers
> diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
> index 2a9b4fe4a84e..d70315fddef6 100644
> --- a/tools/memory-model/linux-kernel.cat
> +++ b/tools/memory-model/linux-kernel.cat
> @@ -27,7 +27,7 @@ include "lock.cat"
> (* Release Acquire *)
> let acq-po = [Acquire] ; po ; [M]
> let po-rel = [M] ; po ; [Release]
> -let po-unlock-rf-lock-po = po ; [UL] ; rf ; [LKR] ; po
> +let po-unlock-lock-po = po ; [UL] ; (po|rf) ; [LKR] ; po
>
> (* Fences *)
> let R4rmb = R \ Noreturn (* Reads for which rmb works *)
> @@ -70,12 +70,12 @@ let rwdep = (dep | ctrl) ; [W]
> let overwrite = co | fr
> let to-w = rwdep | (overwrite & int) | (addr ; [Plain] ; wmb)
> let to-r = addr | (dep ; [Marked] ; rfi)
> -let ppo = to-r | to-w | fence | (po-unlock-rf-lock-po & int)
> +let ppo = to-r | to-w | fence | (po-unlock-lock-po & int)
>
> (* Propagation: Ordering from release operations and strong fences. *)
> let A-cumul(r) = (rfe ; [Marked])? ; r
> let cumul-fence = [Marked] ; (A-cumul(strong-fence | po-rel) | wmb |
> - po-unlock-rf-lock-po) ; [Marked]
> + po-unlock-lock-po) ; [Marked]
> let prop = [Marked] ; (overwrite & ext)? ; cumul-fence* ;
> [Marked] ; rfe? ; [Marked]
>
> diff --git a/tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus b/tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus
> new file mode 100644
> index 000000000000..955b9c7cdc7f
> --- /dev/null
> +++ b/tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus
> @@ -0,0 +1,33 @@
> +C LB+unlocklockonceonce+poacquireonce
> +
> +(*
> + * Result: Never
> + *
> + * If two locked critical sections execute on the same CPU, all accesses
> + * in the first must execute before any accesses in the second, even if
> + * the critical sections are protected by different locks.
> + *)
> +
> +{}
> +
> +P0(spinlock_t *s, spinlock_t *t, int *x, int *y)
> +{
> + int r1;
> +
> + spin_lock(s);
> + r1 = READ_ONCE(*x);
> + spin_unlock(s);
> + spin_lock(t);
> + WRITE_ONCE(*y, 1);
> + spin_unlock(t);
> +}
> +
> +P1(int *x, int *y)
> +{
> + int r2;
> +
> + r2 = smp_load_acquire(y);
> + WRITE_ONCE(*x, 1);
> +}
> +
> +exists (0:r1=1 /\ 1:r2=1)
> diff --git a/tools/memory-model/litmus-tests/MP+unlocklockonceonce+fencermbonceonce.litmus b/tools/memory-model/litmus-tests/MP+unlocklockonceonce+fencermbonceonce.litmus
> new file mode 100644
> index 000000000000..2feb1398be71
> --- /dev/null
> +++ b/tools/memory-model/litmus-tests/MP+unlocklockonceonce+fencermbonceonce.litmus
> @@ -0,0 +1,33 @@
> +C MP+unlocklockonceonce+fencermbonceonce
> +
> +(*
> + * Result: Never
> + *
> + * If two locked critical sections execute on the same CPU, stores in the
> + * first must propagate to each CPU before stores in the second do, even if
> + * the critical sections are protected by different locks.
> + *)
> +
> +{}
> +
> +P0(spinlock_t *s, spinlock_t *t, int *x, int *y)
> +{
> + spin_lock(s);
> + WRITE_ONCE(*x, 1);
> + spin_unlock(s);
> + spin_lock(t);
> + WRITE_ONCE(*y, 1);
> + spin_unlock(t);
> +}
> +
> +P1(int *x, int *y)
> +{
> + int r1;
> + int r2;
> +
> + r1 = READ_ONCE(*y);
> + smp_rmb();
> + r2 = READ_ONCE(*x);
> +}
> +
> +exists (1:r1=1 /\ 1:r2=0)
> diff --git a/tools/memory-model/litmus-tests/README b/tools/memory-model/litmus-tests/README
> index 681f9067fa9e..d311a0ff1ae6 100644
> --- a/tools/memory-model/litmus-tests/README
> +++ b/tools/memory-model/litmus-tests/README
> @@ -63,6 +63,10 @@ LB+poonceonces.litmus
> As above, but with store-release replaced with WRITE_ONCE()
> and load-acquire replaced with READ_ONCE().
>
> +LB+unlocklockonceonce+poacquireonce.litmus
> + Does a unlock+lock pair provides ordering guarantee between a
> + load and a store?
> +
> MP+onceassign+derefonce.litmus
> As below, but with rcu_assign_pointer() and an rcu_dereference().
>
> @@ -90,6 +94,10 @@ MP+porevlocks.litmus
> As below, but with the first access of the writer process
> and the second access of reader process protected by a lock.
>
> +MP+unlocklockonceonce+fencermbonceonce.litmus
> + Does a unlock+lock pair provides ordering guarantee between a
> + store and another store?
> +
> MP+fencewmbonceonce+fencermbonceonce.litmus
> Does a smp_wmb() (between the stores) and an smp_rmb() (between
> the loads) suffice for the message-passing litmus test, where one
> --
> 2.32.0
>
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