linux-kernel - Re: [PATCH 2/3] futex: Use RCU-based per-CPU reference counting instead of rcuref

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Message-ID: <20250708065124.DtoAH-O1@linutronix.de>
Date: Tue, 8 Jul 2025 08:51:24 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org
Cc: André Almeida <andrealmeid@...lia.com>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH 2/3] futex: Use RCU-based per-CPU reference counting
 instead of rcuref_t

On 2025-07-07 16:36:22 [+0200], To linux-kernel@...r.kernel.org wrote:
> From: Peter Zijlstra <peterz@...radead.org>
> 
> The use of rcuref_t for reference counting introduces a performance bottleneck
> when accessed concurrently by multiple threads during futex operations.

just folded this bit after kernel test robot complained:

diff --git a/include/linux/futex.h b/include/linux/futex.h
index cd773febd497b..9e9750f049805 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -113,7 +113,7 @@ static inline int futex_hash_allocate_default(void)
 	return 0;
 }
 static inline int futex_hash_free(struct mm_struct *mm) { return 0; }
-static inline void futex_mm_init(struct mm_struct *mm) { }
+static inline int futex_mm_init(struct mm_struct *mm) { return 0; }
 
 #endif
 

Sebastian