[RFC PATCH 1/2] riscv/spinlock: Strengthen implementations with fences

Palmer Dabbelt palmer at sifive.com
Tue Mar 6 18:02:28 PST 2018 

On Mon, 05 Mar 2018 10:24:09 PST (-0800), parri.andrea at gmail.com wrote:
> Current implementations map locking operations using .rl and .aq
> annotations.  However, this mapping is unsound w.r.t. the kernel
> memory consistency model (LKMM) [1]:
>
> Referring to the "unlock-lock-read-ordering" test reported below,
> Daniel wrote:
>
>   "I think an RCpc interpretation of .aq and .rl would in fact
>    allow the two normal loads in P1 to be reordered [...]
>
>    The intuition would be that the amoswap.w.aq can forward from
>    the amoswap.w.rl while that's still in the store buffer, and
>    then the lw x3,0(x4) can also perform while the amoswap.w.rl
>    is still in the store buffer, all before the l1 x1,0(x2)
>    executes.  That's not forbidden unless the amoswaps are RCsc,
>    unless I'm missing something.
>
>    Likewise even if the unlock()/lock() is between two stores.
>    A control dependency might originate from the load part of
>    the amoswap.w.aq, but there still would have to be something
>    to ensure that this load part in fact performs after the store
>    part of the amoswap.w.rl performs globally, and that's not
>    automatic under RCpc."
>
> Simulation of the RISC-V memory consistency model confirmed this
> expectation.
>
> In order to "synchronize" LKMM and RISC-V's implementation, this
> commit strengthens the implementations of the locking operations
> by replacing .rl and .aq with the use of ("lightweigth") fences,
> resp., "fence rw,  w" and "fence r , rw".
>
> 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(*x);
> }
>
> exists (1:r0=1 /\ 1:r1=0)
>
> [1] https://marc.info/?l=linux-kernel&m=151930201102853&w=2
>     https://groups.google.com/a/groups.riscv.org/forum/#!topic/isa-dev/hKywNHBkAXM
>     https://marc.info/?l=linux-kernel&m=151633436614259&w=2
>
> Signed-off-by: Andrea Parri <parri.andrea at gmail.com>
> Cc: Palmer Dabbelt <palmer at sifive.com>
> Cc: Albert Ou <albert at sifive.com>
> Cc: Daniel Lustig <dlustig at nvidia.com>
> Cc: Alan Stern <stern at rowland.harvard.edu>
> Cc: Will Deacon <will.deacon at arm.com>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Boqun Feng <boqun.feng at gmail.com>
> Cc: Nicholas Piggin <npiggin at gmail.com>
> Cc: David Howells <dhowells at redhat.com>
> Cc: Jade Alglave <j.alglave at ucl.ac.uk>
> Cc: Luc Maranget <luc.maranget at inria.fr>
> Cc: "Paul E. McKenney" <paulmck at linux.vnet.ibm.com>
> Cc: Akira Yokosawa <akiyks at gmail.com>
> Cc: Ingo Molnar <mingo at kernel.org>
> Cc: Linus Torvalds <torvalds at linux-foundation.org>
> Cc: linux-riscv at lists.infradead.org
> Cc: linux-kernel at vger.kernel.org
> ---
>  arch/riscv/include/asm/fence.h    | 12 ++++++++++++
>  arch/riscv/include/asm/spinlock.h | 29 +++++++++++++++--------------
>  2 files changed, 27 insertions(+), 14 deletions(-)
>  create mode 100644 arch/riscv/include/asm/fence.h

Oh, sorry about this -- I thought I'd deleted all this code, but I guess I just 
wrote a patch and then forgot about it.  Here's my original patch, which I have 
marked as a WIP:

commit 39908f1f8b75ae88ce44dc77b8219a94078ad298
Author: Palmer Dabbelt <palmer at sifive.com>
Date:   Tue Dec 5 16:26:50 2017 -0800

    RISC-V: Use generic spin and rw locks

    This might not be exactly the right thing to do: we could use LR/SC to
    produce slightly better locks by rolling the tests into the LR/SC.  I'm
    going to defer that until I get a better handle on the new memory model
    and just be safe here: after some discussion I'm pretty sure the AMOs
    are good, and cmpxchg is safe (by being way too string).

    Since we'd want to rewrite the spinlocks anyway so they queue, I don't
    see any reason to keep the old implementations around.

    Signed-off-by: Palmer Dabbelt <palmer at sifive.com>

diff --git a/arch/riscv/include/asm/spinlock.h b/arch/riscv/include/asm/spinlock.h
index 2fd27e8ef1fd..9b166ea81fe5 100644
--- a/arch/riscv/include/asm/spinlock.h
+++ b/arch/riscv/include/asm/spinlock.h
@@ -15,128 +15,7 @@
 #ifndef _ASM_RISCV_SPINLOCK_H
 #define _ASM_RISCV_SPINLOCK_H

-#include <linux/kernel.h>
-#include <asm/current.h>
-
-/*
- * Simple spin lock operations.  These provide no fairness guarantees.
- */
-
-/* FIXME: Replace this with a ticket lock, like MIPS. */
-
-#define arch_spin_is_locked(x)	(READ_ONCE((x)->lock) != 0)
-
-static inline void arch_spin_unlock(arch_spinlock_t *lock)
-{
-	__asm__ __volatile__ (
-		"amoswap.w.rl x0, x0, %0"
-		: "=A" (lock->lock)
-		:: "memory");
-}
-
-static inline int arch_spin_trylock(arch_spinlock_t *lock)
-{
-	int tmp = 1, busy;
-
-	__asm__ __volatile__ (
-		"amoswap.w.aq %0, %2, %1"
-		: "=r" (busy), "+A" (lock->lock)
-		: "r" (tmp)
-		: "memory");
-
-	return !busy;
-}
-
-static inline void arch_spin_lock(arch_spinlock_t *lock)
-{
-	while (1) {
-		if (arch_spin_is_locked(lock))
-			continue;
-
-		if (arch_spin_trylock(lock))
-			break;
-	}
-}
-
-/***********************************************************/
-
-static inline void arch_read_lock(arch_rwlock_t *lock)
-{
-	int tmp;
-
-	__asm__ __volatile__(
-		"1:	lr.w	%1, %0\n"
-		"	bltz	%1, 1b\n"
-		"	addi	%1, %1, 1\n"
-		"	sc.w.aq	%1, %1, %0\n"
-		"	bnez	%1, 1b\n"
-		: "+A" (lock->lock), "=&r" (tmp)
-		:: "memory");
-}
-
-static inline void arch_write_lock(arch_rwlock_t *lock)
-{
-	int tmp;
-
-	__asm__ __volatile__(
-		"1:	lr.w	%1, %0\n"
-		"	bnez	%1, 1b\n"
-		"	li	%1, -1\n"
-		"	sc.w.aq	%1, %1, %0\n"
-		"	bnez	%1, 1b\n"
-		: "+A" (lock->lock), "=&r" (tmp)
-		:: "memory");
-}
-
-static inline int arch_read_trylock(arch_rwlock_t *lock)
-{
-	int busy;
-
-	__asm__ __volatile__(
-		"1:	lr.w	%1, %0\n"
-		"	bltz	%1, 1f\n"
-		"	addi	%1, %1, 1\n"
-		"	sc.w.aq	%1, %1, %0\n"
-		"	bnez	%1, 1b\n"
-		"1:\n"
-		: "+A" (lock->lock), "=&r" (busy)
-		:: "memory");
-
-	return !busy;
-}
-
-static inline int arch_write_trylock(arch_rwlock_t *lock)
-{
-	int busy;
-
-	__asm__ __volatile__(
-		"1:	lr.w	%1, %0\n"
-		"	bnez	%1, 1f\n"
-		"	li	%1, -1\n"
-		"	sc.w.aq	%1, %1, %0\n"
-		"	bnez	%1, 1b\n"
-		"1:\n"
-		: "+A" (lock->lock), "=&r" (busy)
-		:: "memory");
-
-	return !busy;
-}
-
-static inline void arch_read_unlock(arch_rwlock_t *lock)
-{
-	__asm__ __volatile__(
-		"amoadd.w.rl x0, %1, %0"
-		: "+A" (lock->lock)
-		: "r" (-1)
-		: "memory");
-}
-
-static inline void arch_write_unlock(arch_rwlock_t *lock)
-{
-	__asm__ __volatile__ (
-		"amoswap.w.rl x0, x0, %0"
-		: "=A" (lock->lock)
-		:: "memory");
-}
+#include <asm-generic/qspinlock.h>
+#include <asm-generic/qrwlock.h>

 #endif /* _ASM_RISCV_SPINLOCK_H */

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