linux-kernel - Re: [PATCH v4 0/5] barrier: Add smp_cond_load_*

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

Message-ID: <87tt1kpj4z.fsf@oracle.com>
Date: Tue, 02 Sep 2025 15:46:52 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        bpf@...r.kernel.org, arnd@...db.de, will@...nel.org,
        peterz@...radead.org, akpm@...ux-foundation.org, mark.rutland@....com,
        harisokn@...zon.com, cl@...two.org, ast@...nel.org, memxor@...il.com,
        zhenglifeng1@...wei.com, xueshuai@...ux.alibaba.com,
        joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com
Subject: Re: [PATCH v4 0/5] barrier: Add smp_cond_load_*_timewait()


Catalin Marinas <catalin.marinas@....com> writes:

> On Fri, Aug 29, 2025 at 01:07:30AM -0700, Ankur Arora wrote:
>> Ankur Arora (5):
>>   asm-generic: barrier: Add smp_cond_load_relaxed_timewait()
>>   arm64: barrier: Add smp_cond_load_relaxed_timewait()
>>   arm64: rqspinlock: Remove private copy of
>>     smp_cond_load_acquire_timewait
>>   asm-generic: barrier: Add smp_cond_load_acquire_timewait()
>>   rqspinlock: use smp_cond_load_acquire_timewait()
>
> Can you have a go at poll_idle() to see how it would look like using
> this API? It doesn't necessarily mean we have to merge them all at once
> but it gives us a better idea of the suitability of the interface.

So, I've been testing with some version of the following:

diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
index 9b6d90a72601..361879396d0c 100644
--- a/drivers/cpuidle/poll_state.c
+++ b/drivers/cpuidle/poll_state.c
@@ -8,35 +8,25 @@
 #include <linux/sched/clock.h>
 #include <linux/sched/idle.h>

-#define POLL_IDLE_RELAX_COUNT	200
-
 static int __cpuidle poll_idle(struct cpuidle_device *dev,
 			       struct cpuidle_driver *drv, int index)
 {
-	u64 time_start;
-
-	time_start = local_clock_noinstr();
+	unsigned long flags;

 	dev->poll_time_limit = false;

 	raw_local_irq_enable();
 	if (!current_set_polling_and_test()) {
-		unsigned int loop_count = 0;
-		u64 limit;
+		u64 limit, time_end;

 		limit = cpuidle_poll_time(drv, dev);
+		time_end = local_clock_noinstr() + limit;

-		while (!need_resched()) {
-			cpu_relax();
-			if (loop_count++ < POLL_IDLE_RELAX_COUNT)
-				continue;
+		flags = smp_cond_load_relaxed_timewait(&current_thread_info()->flags,
+						       VAL & _TIF_NEED_RESCHED,
+						       (local_clock_noinstr() >= time_end));

-			loop_count = 0;
-			if (local_clock_noinstr() - time_start > limit) {
-				dev->poll_time_limit = true;
-				break;
-			}
-		}
+		dev->poll_time_limit = (local_clock_noinstr() >= time_end);
 	}
 	raw_local_irq_disable();

With that, poll_idle() is:

  static int __cpuidle poll_idle(struct cpuidle_device *dev,
			       struct cpuidle_driver *drv, int index)
  {
	unsigned long flags;

	dev->poll_time_limit = false;

	raw_local_irq_enable();
	if (!current_set_polling_and_test()) {
		u64 limit, time_end;

		limit = cpuidle_poll_time(drv, dev);
		time_end = local_clock_noinstr() + limit;

		flags = smp_cond_load_relaxed_timewait(&current_thread_info()->flags,
						       VAL & _TIF_NEED_RESCHED,
						       (local_clock_noinstr() >= time_end));

		dev->poll_time_limit = (local_clock_noinstr() >= time_end);
	}
	raw_local_irq_disable();

	current_clr_polling();

	return index;
  }

--
ankur