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Subject[RFC PATCH] riscv/locking: Strengthen spin_lock() and spin_unlock()
Date
The LKMM defines certain memory ordering constraints for spin_lock(),
spin_unlock() and other primitives that the kernel developer can rely
on; unfortunately, some of these constraints are not currently met by
the RISC-V implementation of spin_lock(), spin_unlock().

The following MP-like program exemplifies the issue: according to our
LKMM, program "unlock-lock-read-ordering" below can never reach state
(1:r0=1 /\ 1:r1=0). However, when we map this C program to the RISCV
program "RISCV-unlock-lock-read-ordering" below following the current
implementation, the corresponding state is reachable according to the
RISCV specification and its formalizations [2].

C unlock-lock-read-ordering

{}
/* s initially owned by P1 */

P0(int *x, int *y)
{
WRITE_ONCE(*x, 1);
smp_wmb();
WRITE_ONCE(*y, 1);
}

P1(int *x, int *y, spinlock_t *s)
{
int r0;
int r1;

r0 = READ_ONCE(*y);
spin_unlock(s);
spin_lock(s);
r1 = READ_ONCE(*y);
}

exists (1:r0=1 /\ 1:r1=0)

RISCV RISCV-unlock-lock-read-ordering
{
0:x2=x; 0:x4=y;
1:x2=y; 1:x4=x; 1:x6=s;
s=1;
}
P0 | P1 ;
ori x1,x0,1 | lw x1,0(x2) ;
sw x1,0(x2) | amoswap.w.rl x0,x0,(x6) ;
fence w,w | ori x5,x0,1 ;
ori x3,x0,1 | amoswap.w.aq x0,x5,(x6) ;
sw x3,0(x4) | lw x3,0(x4) ;
exists
(1:x1=1 /\ 1:x3=0)

The issue can in fact be exarcebated if, as envisaged/discussed in [3],
the LKMM will be modified to become even more "demanding" on the order-
ing constraints associated to the locking primitives. For example the
state (1:r0=1 /\ 1:r1=0) in program "unlock-lock-write-ordering" below
is currently allowed by LKMM (as is the corresponding state in "RISCV-
unlock-lock-write-ordering" below). However, proposals modifying LKMM
to _forbid_ that state have already appeared on LKML [4].

C unlock-lock-write-ordering

{}
/* s initially owned by P0 */

P0(int *x, int *y, spinlock_t *s)
{
WRITE_ONCE(*x, 1);
spin_unlock(s);
spin_lock(s);
WRITE_ONCE(*y, 1);
}

P1(int *x, int *y)
{
int r0;
int r1;

r0 = READ_ONCE(*y);
smp_rmb();
r1 = READ_ONCE(*y);
}

exists (1:r0=1 /\ 1:r1=0)

RISCV RISCV-unlock-lock-write-ordering
{
0:x2=x; 0:x4=y; 0:x6=s;
1:x2=y; 1:x4=x;
s=1;
}
P0 | P1 ;
ori x1,x0,1 | lw x1,0(x2) ;
sw x1,0(x2) | fence r,r ;
amoswap.w.rl x0,x0,(x6) | lw x3,0(x4) ;
ori x5,x0,1 | ;
amoswap.w.aq x0,x5,(x6) | ;
ori x3,x0,1 | ;
sw x3,0(x4) | ;
exists
(1:x1=1 /\ 1:x3=0)

[Curiously, RISC-V's current implementations of smp_load_acquire() and
smp_store_release() provide way stronger ordering than what currently
required by LKMM since those're relying on the generic implementation
(c.f, also, [5]). ]

This RFC fixes the issue by strengthening RISC-V's implementations of
spin_lock() and spin_unlock(), based on "A spinlock with fences" from
Section 2.3.5 ("Acquire/Release Ordering") of the RISC-V draft spec.
It does _not_ attempt to fix read-lock and atomics (for which, AFAICT,
similar considerations would hold).

IMPORTANT. This patch is _NOT_ intended to be applied as is. Rather,
this is intended to test the waters, implicit questions being "Should
we take this direction?" "Are changes to LKMM needed?" (and develop a
technical discussion on the above issues.)

[1] https://marc.info/?l=linux-kernel&m=151633436614259&w=2
[2] https://groups.google.com/a/groups.riscv.org/forum/#!topic/isa-dev/hKywNHBkAXM
[3] https://marc.info/?l=linux-kernel&m=151181741400461&w=2
[4] https://marc.info/?l=linux-kernel&m=151871035014425&w=2
[5] https://marc.info/?l=linux-kernel&m=151912186913692&w=2

Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Albert Ou <albert@sifive.com>
Cc: Daniel Lustig <dlustig@nvidia.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jade Alglave <j.alglave@ucl.ac.uk>
Cc: Luc Maranget <luc.maranget@inria.fr>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-riscv@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
---
arch/riscv/include/asm/spinlock.h | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/include/asm/spinlock.h b/arch/riscv/include/asm/spinlock.h
index 2fd27e8ef1fd6..2f89fc62c9196 100644
--- a/arch/riscv/include/asm/spinlock.h
+++ b/arch/riscv/include/asm/spinlock.h
@@ -28,8 +28,9 @@

static inline void arch_spin_unlock(arch_spinlock_t *lock)
{
+ RISCV_FENCE(rw,w);
__asm__ __volatile__ (
- "amoswap.w.rl x0, x0, %0"
+ "amoswap.w x0, x0, %0"
: "=A" (lock->lock)
:: "memory");
}
@@ -39,10 +40,11 @@ static inline int arch_spin_trylock(arch_spinlock_t *lock)
int tmp = 1, busy;

__asm__ __volatile__ (
- "amoswap.w.aq %0, %2, %1"
+ "amoswap.w %0, %2, %1"
: "=r" (busy), "+A" (lock->lock)
: "r" (tmp)
: "memory");
+ RISCV_FENCE(r,rw);

return !busy;
}
--
2.7.4
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