linux-kernel - [RFC PATCH] wait: avoid spurious calls to prepare_to_wait_event() in ___wait

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [day] [month] [year] [list]

Message-ID: <20250227233652.142613-1-mjguzik@gmail.com>
Date: Fri, 28 Feb 2025 00:36:52 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: mingo@...hat.com,
	peterz@...radead.org,
	juri.lelli@...hat.com,
	rostedt@...dmis.org
Cc: vschneid@...hat.com,
	mgorman@...e.de,
	bsegall@...gle.com,
	dietmar.eggemann@....com,
	vincent.guittot@...aro.org,
	linux-kernel@...r.kernel.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [RFC PATCH] wait: avoid spurious calls to prepare_to_wait_event() in  ___wait_event()

In vast majority of cases the condition determining whether the thread
can proceed is true after the first wake up.

However, even in that there is an additional call to prepare_to_wait_event(),
resulting in a spurious irq + lock trip.

Then it calls into finish_wait() to unlink itself.

pre-check the condition instead after waking up instead.

Stats gathared during a kernel build:
bpftrace -e 'kprobe:prepare_to_wait_event,kprobe:finish_wait \
		 { @[probe] = count(); }'

@[kprobe:finish_wait]: 392483
@[kprobe:prepare_to_wait_event]: 778690

As in calls to prepare_to_wait_event() almost double calls to
finish_wait(). This evens out with the patch.

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---

One may worry about using "condition" twice. However, macros leading up
to this one already do it, so it should be fine.

Also one may wonder about fences -- to my understanding going off and on
CPU guarantees a full fence.

 include/linux/wait.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/wait.h b/include/linux/wait.h
index 2bdc8f47963b..965a19809c7e 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -316,6 +316,9 @@ extern void init_wait_entry(struct wait_queue_entry *wq_entry, int flags);
 		}								\
 										\
 		cmd;								\
+										\
+		if (condition)							\
+			break;							\
 	}									\
 	finish_wait(&wq_head, &__wq_entry);					\
 __out:	__ret;									\
-- 
2.43.0