Re: [PATCH 10/17] RISC-V: Atomic and Locking Code

[Boqun Feng]

On Tue, Jul 11, 2017 at 06:31:23PM -0700, Palmer Dabbelt wrote:
[...]
> diff --git a/arch/riscv/include/asm/bitops.h b/arch/riscv/include/asm/bitops.h
> new file mode 100644
> index 000000000000..b0a0c76e966a
> --- /dev/null
> +++ b/arch/riscv/include/asm/bitops.h
> @@ -0,0 +1,216 @@
> +/*
> + * Copyright (C) 2012 Regents of the University of California
> + *
> + *   This program is free software; you can redistribute it and/or
> + *   modify it under the terms of the GNU General Public License
> + *   as published by the Free Software Foundation, version 2.
> + *
> + *   This program is distributed in the hope that it will be useful,
> + *   but WITHOUT ANY WARRANTY; without even the implied warranty of
> + *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + *   GNU General Public License for more details.
> + */
> +
> +#ifndef _ASM_RISCV_BITOPS_H
> +#define _ASM_RISCV_BITOPS_H
> +
> +#ifndef _LINUX_BITOPS_H
> +#error "Only <linux/bitops.h> can be included directly"
> +#endif /* _LINUX_BITOPS_H */
> +
> +#include <linux/compiler.h>
> +#include <linux/irqflags.h>
> +#include <asm/barrier.h>
> +#include <asm/bitsperlong.h>
> +
> +#ifndef smp_mb__before_clear_bit
> +#define smp_mb__before_clear_bit()  smp_mb()
> +#define smp_mb__after_clear_bit()   smp_mb()
> +#endif /* smp_mb__before_clear_bit */
> +
> +#include <asm-generic/bitops/__ffs.h>
> +#include <asm-generic/bitops/ffz.h>
> +#include <asm-generic/bitops/fls.h>
> +#include <asm-generic/bitops/__fls.h>
> +#include <asm-generic/bitops/fls64.h>
> +#include <asm-generic/bitops/find.h>
> +#include <asm-generic/bitops/sched.h>
> +#include <asm-generic/bitops/ffs.h>
> +
> +#include <asm-generic/bitops/hweight.h>
> +
> +#if (BITS_PER_LONG == 64)
> +#define __AMO(op)	"amo" #op ".d"
> +#elif (BITS_PER_LONG == 32)
> +#define __AMO(op)	"amo" #op ".w"
> +#else
> +#error "Unexpected BITS_PER_LONG"
> +#endif
> +

So the test_and_{set,change,clear}_bit() have the similar semantics as
cmpxchg(), so

> +#define __test_and_op_bit(op, mod, nr, addr)			\
> +({								\
> +	unsigned long __res, __mask;				\
> +	__mask = BIT_MASK(nr);					\
> +	__asm__ __volatile__ (					\
> +		__AMO(op) " %0, %2, %1"				\

... "aqrl" bit is needed here, and

> +		: "=r" (__res), "+A" (addr[BIT_WORD(nr)])	\
> +		: "r" (mod(__mask)));				\

... "memory" clobber is needed here.

> +	((__res & __mask) != 0);				\
> +})
> +
> +#define __op_bit(op, mod, nr, addr)				\
> +	__asm__ __volatile__ (					\
> +		__AMO(op) " zero, %1, %0"			\
> +		: "+A" (addr[BIT_WORD(nr)])			\
> +		: "r" (mod(BIT_MASK(nr))))
> +
> +/* Bitmask modifiers */
> +#define __NOP(x)	(x)
> +#define __NOT(x)	(~(x))
> +
> +/**
> + * test_and_set_bit - Set a bit and return its old value
> + * @nr: Bit to set
> + * @addr: Address to count from
> + *
> + * This operation is atomic and cannot be reordered.
> + * It may be reordered on other architectures than x86.
> + * It also implies a memory barrier.
> + */
> +static inline int test_and_set_bit(int nr, volatile unsigned long *addr)
> +{
> +	return __test_and_op_bit(or, __NOP, nr, addr);
> +}
> +
> +/**
> + * test_and_clear_bit - Clear a bit and return its old value
> + * @nr: Bit to clear
> + * @addr: Address to count from
> + *
> + * This operation is atomic and cannot be reordered.
> + * It can be reordered on other architectures other than x86.
> + * It also implies a memory barrier.
> + */
> +static inline int test_and_clear_bit(int nr, volatile unsigned long *addr)
> +{
> +	return __test_and_op_bit(and, __NOT, nr, addr);
> +}
> +
> +/**
> + * test_and_change_bit - Change a bit and return its old value
> + * @nr: Bit to change
> + * @addr: Address to count from
> + *
> + * This operation is atomic and cannot be reordered.
> + * It also implies a memory barrier.
> + */
> +static inline int test_and_change_bit(int nr, volatile unsigned long *addr)
> +{
> +	return __test_and_op_bit(xor, __NOP, nr, addr);
> +}
> +
> +/**
> + * set_bit - Atomically set a bit in memory
> + * @nr: the bit to set
> + * @addr: the address to start counting from
> + *
> + * This function is atomic and may not be reordered.  See __set_bit()

This is incorrect, {set,change,clear}_bit() can be reordered, see
Documentation/memory-barriers.txt, they are just relaxed atomics. But I
think you just copy this from x86 code, so maybe x86 code needs help
too, at least claim that's only x86-specific guarantee.

> + * if you do not require the atomic guarantees.
> + *
> + * Note: there are no guarantees that this function will not be reordered
> + * on non x86 architectures, so if you are writing portable code,
> + * make sure not to rely on its reordering guarantees.
> + *
> + * Note that @nr may be almost arbitrarily large; this function is not
> + * restricted to acting on a single-word quantity.
> + */
> +static inline void set_bit(int nr, volatile unsigned long *addr)
> +{
> +	__op_bit(or, __NOP, nr, addr);
> +}
> +
> +/**
> + * clear_bit - Clears a bit in memory
> + * @nr: Bit to clear
> + * @addr: Address to start counting from
> + *
> + * clear_bit() is atomic and may not be reordered.  However, it does
> + * not contain a memory barrier, so if it is used for locking purposes,
> + * you should call smp_mb__before_clear_bit() and/or smp_mb__after_clear_bit()
> + * in order to ensure changes are visible on other processors.
> + */
> +static inline void clear_bit(int nr, volatile unsigned long *addr)
> +{
> +	__op_bit(and, __NOT, nr, addr);
> +}
> +
> +/**
> + * change_bit - Toggle a bit in memory
> + * @nr: Bit to change
> + * @addr: Address to start counting from
> + *
> + * change_bit() is atomic and may not be reordered. It may be
> + * reordered on other architectures than x86.
> + * Note that @nr may be almost arbitrarily large; this function is not
> + * restricted to acting on a single-word quantity.
> + */
> +static inline void change_bit(int nr, volatile unsigned long *addr)
> +{
> +	__op_bit(xor, __NOP, nr, addr);
> +}
> +
> +/**
> + * test_and_set_bit_lock - Set a bit and return its old value, for lock
> + * @nr: Bit to set
> + * @addr: Address to count from
> + *
> + * This operation is atomic and provides acquire barrier semantics.
> + * It can be used to implement bit locks.
> + */
> +static inline int test_and_set_bit_lock(
> +	unsigned long nr, volatile unsigned long *addr)
> +{
> +	return test_and_set_bit(nr, addr);

If you want, you can open code an "amoor.aq" here, because
test_and_set_bit_lock() only needs an acquire barrier.

> +}
> +
> +/**
> + * clear_bit_unlock - Clear a bit in memory, for unlock
> + * @nr: the bit to set
> + * @addr: the address to start counting from
> + *
> + * This operation is atomic and provides release barrier semantics.
> + */
> +static inline void clear_bit_unlock(
> +	unsigned long nr, volatile unsigned long *addr)
> +{

You need a smp_mb__before_atomic() here, because clear_bit() is only
relaxed atomic. And clear_bit_unlock() is a release.

Regards,
Boqun

> +	clear_bit(nr, addr);
> +}
> +
> +/**
> + * __clear_bit_unlock - Clear a bit in memory, for unlock
> + * @nr: the bit to set
> + * @addr: the address to start counting from
> + *
> + * This operation is like clear_bit_unlock, however it is not atomic.
> + * It does provide release barrier semantics so it can be used to unlock
> + * a bit lock, however it would only be used if no other CPU can modify
> + * any bits in the memory until the lock is released (a good example is
> + * if the bit lock itself protects access to the other bits in the word).
> + */
> +static inline void __clear_bit_unlock(
> +	unsigned long nr, volatile unsigned long *addr)
> +{
> +	clear_bit(nr, addr);
> +}
> +
> +#undef __test_and_op_bit
> +#undef __op_bit
> +#undef __NOP
> +#undef __NOT
> +#undef __AMO
> +
> +#include <asm-generic/bitops/non-atomic.h>
> +#include <asm-generic/bitops/le.h>
> +#include <asm-generic/bitops/ext2-atomic.h>
> +
> +#endif /* _ASM_RISCV_BITOPS_H */
[...]

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