linux-kernel - Re: [PATCH 1/7] cleanup: Introduce DEFINE_ACQUIRE() a CLASS() for conditional locking

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Message-ID: <6822bd346a0f4_49706100a6@dwillia2-mobl4.notmuch>
Date: Mon, 12 May 2025 20:32:04 -0700
From: <dan.j.williams@...el.com>
To: Peter Zijlstra <peterz@...radead.org>, <dan.j.williams@...el.com>
CC: <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>, David Lechner
	<dlechner@...libre.com>, Linus Torvalds <torvalds@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...nel.org>, "Fabio M. De Francesco"
	<fabio.maria.de.francesco@...ux.intel.com>, Davidlohr Bueso
	<dave@...olabs.net>, Jonathan Cameron <jonathan.cameron@...wei.com>, "Dave
 Jiang" <dave.jiang@...el.com>, Alison Schofield <alison.schofield@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH 1/7] cleanup: Introduce DEFINE_ACQUIRE() a CLASS() for
 conditional locking

Peter Zijlstra wrote:
> On Mon, May 12, 2025 at 08:25:59PM +0200, Peter Zijlstra wrote:
> > On Mon, May 12, 2025 at 12:50:26PM +0200, Peter Zijlstra wrote:
> > 
> > > +#define __GUARD_ERR(_ptr) \
> > > +	({ long _rc = (__force unsigned long)(_ptr); \
> > > +	   if (!_rc) { _rc = -EBUSY; } if (!IS_ERR_VALUE(_rc)) { _rc = 0; } \
> > > +	   _rc; })
> > > +
> > 
> > >  #define DEFINE_GUARD(_name, _type, _lock, _unlock) \
> > > -	DEFINE_CLASS(_name, _type, if (_T) { _unlock; }, ({ _lock; _T; }), _type _T); \
> > > +	DEFINE_CLASS(_name, _type, if (!__GUARD_ERR(_T)) { _unlock; }, ({ _lock; _T; }), _type _T); \
> > >  	DEFINE_CLASS_IS_GUARD(_name)
> > 
> > GCC is 'stupid' and this generates atrocious code. I'll play with it.
> 
> PRE:
>     bf9e:       48 85 db                test   %rbx,%rbx
>     bfa1:       74 1a                   je     bfbd <foo+0x5d>
>     bfa3:       48 81 fb 00 f0 ff ff    cmp    $0xfffffffffffff000,%rbx
>     bfaa:       77 11                   ja     bfbd <foo+0x5d>
> 
> POST:
>     bf9e:       48 8d 43 ff             lea    -0x1(%rbx),%rax
>     bfa2:       48 3d ff ef ff ff       cmp    $0xffffffffffffefff,%rax
>     bfa8:       77 11                   ja     bfbb <foo+0x5b>
> 

FWIW this looks good to me, and the conversion to drop all explicit
locking passed tests. I threw it out on a branch to get some bot
coverage in the meantime.

https://git.kernel.org/pub/scm/linux/kernel/git/cxl/cxl.git/log/?h=cxl-acquire