linux-kernel - Re: [PATCH V5 17/30] csky: Misc headers

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Date:   Thu, 27 Sep 2018 13:07:42 +0800
From:   Guo Ren <ren_guo@...ky.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        akpm@...ux-foundation.org, arnd@...db.de,
        daniel.lezcano@...aro.org, davem@...emloft.net,
        gregkh@...uxfoundation.org, jason@...edaemon.net,
        marc.zyngier@....com, mark.rutland@....com,
        mchehab+samsung@...nel.org, robh@...nel.org, robh+dt@...nel.org,
        tglx@...utronix.de, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, devicetree@...r.kernel.org,
        green.hu@...il.com, palmer@...ive.com
Subject: Re: [PATCH V5 17/30] csky: Misc headers

On Tue, Sep 25, 2018 at 12:45:41PM +0200, Peter Zijlstra wrote:
> On Tue, Sep 25, 2018 at 12:08:03PM +0200, Andrea Parri wrote:
> > Hi Guo,
> > 
> > > +/*
> > > + * 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()
> > > + * 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)
> > > +{
> > > +	unsigned long mask = BIT_MASK(nr);
> > > +	unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> > > +	unsigned long tmp;
> > > +
> > > +	/* *p  |= mask; */
> > > +	smp_mb();
> > > +	asm volatile (
> > > +		"1:	ldex.w		%0, (%2)	\n"
> > > +		"	or32		%0, %0, %1	\n"
> > > +		"	stex.w		%0, (%2)	\n"
> > > +		"	bez		%0, 1b		\n"
> > > +		: "=&r"(tmp)
> > > +		: "r"(mask), "r"(p)
> > > +		: "memory");
> > > +	smp_mb();
> > > +}
> > > +
> > > +/**
> > > + * 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_atomic() and/or smp_mb__after_atomic()
> > > + * in order to ensure changes are visible on other processors.
> > > + */
> > > +static inline void clear_bit(int nr, volatile unsigned long *addr)
> > > +{
> > > +	unsigned long mask = BIT_MASK(nr);
> > > +	unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> > > +	unsigned long tmp;
> > > +
> > > +	/* *p &= ~mask; */
> > > +	mask = ~mask;
> > > +	smp_mb();
> > > +	asm volatile (
> > > +		"1:	ldex.w		%0, (%2)	\n"
> > > +		"	and32		%0, %0, %1	\n"
> > > +		"	stex.w		%0, (%2)	\n"
> > > +		"	bez		%0, 1b		\n"
> > > +		: "=&r"(tmp)
> > > +		: "r"(mask), "r"(p)
> > > +		: "memory");
> > > +	smp_mb();
> > > +}
> > > +
> > > +/**
> > > + * 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)
> > > +{
> > > +	unsigned long mask = BIT_MASK(nr);
> > > +	unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> > > +	unsigned long tmp;
> > > +
> > > +	/* *p ^= mask; */
> > > +	smp_mb();
> > > +	asm volatile (
> > > +		"1:	ldex.w		%0, (%2)	\n"
> > > +		"	xor32		%0, %0, %1	\n"
> > > +		"	stex.w		%0, (%2)	\n"
> > > +		"	bez		%0, 1b		\n"
> > > +		: "=&r"(tmp)
> > > +		: "r"(mask), "r"(p)
> > > +		: "memory");
> > > +	smp_mb();
> > > +}
> > 
> > The {set,clear,change}_bit() operations don't have to be ordered: you
> > might want to remove the above smp_mb()s (and adjust the comments).
I confused it with cmpxchg, seems cmpxchg need smp_mb() before&after.
See: https://lkml.org/lkml/2018/7/6/383

> Better yet, you can entirely delete all that and use
> asm-generic/bitops/atomic.h instead.
Yes, approve. Seems I only need care about asm/atomic.h, it's good for
me.

Best Regards
 Guo Ren