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SubjectRe: [RFC PATCH 1/2] riscv/spinlock: Strengthen implementations with fences
On Wed, Mar 07, 2018 at 10:33:49AM -0800, Palmer Dabbelt wrote:

[...]

> I'm going to go produce a new set of spinlocks, I think it'll be a bit more
> coherent then.
>
> I'm keeping your other patch in my queue for now, it generally looks good
> but I haven't looked closely yet.

Patches 1 and 2 address a same issue ("release-to-acquire"); this is also
expressed, more or less explicitly, in the corresponding commit messages:
it might make sense to "queue" them together, and to build the new locks
on top of these (even if this meant "rewrite all of/a large portion of
spinlock.h"...).

Andrea


>
> Thanks!
>
> >
> > Andrea
> >
> >
> >>
> >> Signed-off-by: Palmer Dabbelt <palmer@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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