linux-kernel - Re: [PATCH v2 3/9] atomic: Introduce atomic_{inc,dec,dec_and_test}

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Date:   Mon, 13 Dec 2021 11:57:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     will@...nel.org, boqun.feng@...il.com,
        linux-kernel@...r.kernel.org, x86@...nel.org, elver@...gle.com,
        keescook@...omium.org, hch@...radead.org,
        torvalds@...ux-foundation.org, axboe@...nel.dk
Subject: Re: [PATCH v2 3/9] atomic: Introduce
 atomic_{inc,dec,dec_and_test}_overflow()

On Mon, Dec 13, 2021 at 10:06:01AM +0000, Mark Rutland wrote:
> On Fri, Dec 10, 2021 at 05:16:21PM +0100, Peter Zijlstra wrote:
> > In order to facilitate architecture support for refcount_t, introduce
> > a number of new atomic primitives that have a uaccess style exception
> > for overflow.
> > 
> > Notably:
> > 
> >   atomic_inc_overflow(v, Label):
> > 
> > 	increment and goto Label when the old value of v is zero or
> > 	negative.
> > 
> >   atomic_dec_overflow(v, Label):
> > 
> > 	decrement and goto Label when the new value of v is zero or
> > 	negative
> > 
> >   atomic_dec_and_test_overflow(v, Label):
> > 
> > 	decrement and return true when the result is zero and goto
> > 	Label when the new value of v is negative
> 
> Maybe it's worth adding these as comments in the fallback, which we have for a
> few existing functions, e.g.
> 
> | /**
> |  * arch_${atomic}_add_negative - add and test if negative
> |  * @i: integer value to add 
> |  * @v: pointer of type ${atomic}_t
> |  *
> |  * Atomically adds @i to @v and returns true
> |  * if the result is negative, or false when
> |  * result is greater than or equal to zero.
> |  */
> | static __always_inline bool
> | arch_${atomic}_add_negative(${int} i, ${atomic}_t *v) 
> | {
> |         return arch_${atomic}_add_return(i, v) < 0;
> | }
> 
> Not a big deal either way.

I can do that; but the whole ATOMIC*_OVERFLOW_OFFSET thing complicates
all this somewhat. Still, let me try, I've gotta document that anyway..