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Message-ID: <20250514064624.GA24938@noisy.programming.kicks-ass.net>
Date: Wed, 14 May 2025 08:46:24 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: dan.j.williams@...el.com, linux-cxl@...r.kernel.org,
linux-kernel@...r.kernel.org, David Lechner <dlechner@...libre.com>,
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
On Tue, May 13, 2025 at 12:46:29PM -0700, Linus Torvalds wrote:
> On Tue, 13 May 2025 at 01:50, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > +#define __GUARD_IS_ERR(_ptr) \
> > + ({ unsigned long _var = (__force unsigned long)(_ptr); \
> > + bool _s; \
> > + asm_inline volatile ("cmp %[val], %[var]" \
> > + : "=@...s" (_s) \
> > + : [val] "i" (-MAX_ERRNO), \
> > + [var] "r" (_var)); \
> > + unlikely(_s); })
>
> I think that this might be acceptable if it was some actual common operation.
>
> But for just the conditional guard test, I think it's cute, but I
> don't think it's really worth it.
Its actually every guard, the destructor is shared between unconditional
and conditional locks.
> Right now IS_ERR_OR_NULL() generates pretty disgusting code, with
> clang doing things like this:
>
> testq %rdi, %rdi
> sete %al
> cmpq $-4095, %rdi # imm = 0xF001
> setae %cl
> orb %al, %cl
> je .LBB3_1
Whee, that is creative indeed :-)
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