linux-kernel - Re: [PATCH v2 0/8] cleanup: Introduce ACQUIRE(), a guard() for conditional locks

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Message-ID: <aGXDMZB6omShJpoj@aschofie-mobl2.lan>
Date: Wed, 2 Jul 2025 16:39:29 -0700
From: Alison Schofield <alison.schofield@...el.com>
To: Dan Williams <dan.j.williams@...el.com>
CC: <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Dave Jiang
	<dave.jiang@...el.com>, David Lechner <dlechner@...libre.com>, "Davidlohr
 Bueso" <dave@...olabs.net>, "Fabio M. De Francesco"
	<fabio.maria.de.francesco@...ux.intel.com>, "Fabio M . De Francesco"
	<fabio.m.de.francesco@...ux.intel.com>, Ingo Molnar <mingo@...nel.org>, "Ira
 Weiny" <ira.weiny@...el.com>, Jonathan Cameron <jonathan.cameron@...wei.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra
	<peterz@...radead.org>, Shiju Jose <shiju.jose@...wei.com>, Vishal Verma
	<vishal.l.verma@...el.com>
Subject: Re: [PATCH v2 0/8] cleanup: Introduce ACQUIRE(), a guard() for
 conditional locks

On Wed, Jun 18, 2025 at 10:04:08PM -0700, Dan Williams wrote:
> Changes since v1: [1]
> * Peter took one look at v1 and rewrote it into something significantly
>   better. Unlike my attempt that required suffering a new parallel
>   universe of lock guards, the rewrite reuses existing lock guards.
>   ACQUIRE() can be used any place guard() can be used, and adds
>   ACQUIRE_ERR() to pass the result of conditional locks.
> 
> [1]: http://lore.kernel.org/20250507072145.3614298-1-dan.j.williams@intel.com
> 
> Note, all the code in patch1 is Peter's I just wrapped it in a changelog
> and added some commentary. Peter, forgive me if you were still in the
> process of circling back to this topic. I marked the patch attributed to
> you as: "Not-yet-signed-off-by". Otherwise, my motivation for going
> ahead with a formal submission are the multiple patchsets in my review /
> development queue where I would like to use ACQUIRE().
> 
> The orginal motivation of v1 for this work is that the CXL subsystem
> adopted scope-based helpers and achieved some decent cleanups. However,
> that work stalled with conditional locks. It stalled due to the pain
> points of scoped_cond_guard() detailed in patch1.
> 
> This work also allows for replacing open-coded equivalents like
> rwsem_read_intr_acquire() that went upstream in v6.16:
> 
>     0c6e6f1357cb cxl/edac: Add CXL memory device patrol scrub control feature
> 
> The open question from the discussion [2] was whether it was worth
> defining a __GUARD_IS_ERR() asm helper. I left that alone.
> 
> Lastly, this version of ACQUIRE_ERR() matches Peter's original proposal
> to have the caller pass the lock scope variable by reference [3]. My
> change of heart came from looking at the conversion and wanting
> ACQUIRE_ERR() to be more visually distinct from ACQUIRE() especially
> because it is accessing lock condition metadata, not the lock itself.
> 
> E.g.
> 
>        ACQUIRE(rwsem_read_intr, rwsem)(&cxl_rwsem.region);
>        if ((ret = ACQUIRE_ERR(rwsem_read_intr, &rwsem)))
>                return ret;
> 
> Yes, checkpatch disagrees with assignment in if(), but cleanup.h already
> demands other expections for historical style, and a compact / limited
> idiom for ACQUIRE_ERR() feels reasonable.

Hi Dan,

I've been building upon this set and applying this diff to squelch
those checkpatch ERRORs. Please take a look and consider adding for
review in next version.

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 664f7b7a622c..193a03fa7114 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -5682,7 +5682,14 @@ sub process {
 			my ($s, $c) = ($stat, $cond);
 			my $fixed_assign_in_if = 0;
 
-			if ($c =~ /\bif\s*\(.*[^<>!=]=[^=].*/s) {
+			if ($c =~ /\bif\s*\((.*[^<>!=]=[^=].*)\)/s) {
+				my $expr = $1;
+
+				# Allow ACQUIRE_ERR() special case
+				if ($expr =~ /\w+\s*=\s*ACQUIRE_ERR\s*\(/) {
+					next;
+				}
+
 				if (ERROR("ASSIGN_IN_IF",
 					  "do not use assignment in if condition\n" . $herecurr) &&
 				    $fix && $perl_version_ok) {



snip