linux-kernel - Re: [PATCH 2/4] ARM: Disable HIGHPTE on PREEMPT

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Message-ID: <20241211152257.Igx3aT2Y@linutronix.de>
Date: Wed, 11 Dec 2024 16:22:57 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Linus Walleij <linus.walleij@...aro.org>
Cc: Arnd Bergmann <arnd@...nel.org>, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	linux-mm@...ck.org, linux-rt-devel@...ts.linux.dev,
	Ard Biesheuvel <ardb@...nel.org>,
	Clark Williams <clrkwllms@...nel.org>,
	Jason Baron <jbaron@...mai.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Matthew Wilcox <willy@...radead.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Russell King <linux@...linux.org.uk>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 2/4] ARM: Disable HIGHPTE on PREEMPT_RT kernels

On 2024-12-11 14:29:29 [+0100], Linus Walleij wrote:
> So fast GUP is supposed to be lockless, and should just not
> have this problem. So it can't be addressing gup_fast_pgd_range()
> right?
…
> I'm more asking if HIGHPTE even acquires a spinlock anymore
> as it is supposed to be "fast"/lockless. If it does, it is clearly violating
> the "fast" promise of the fast GUP API and should not exist.

This is lockless on x86. The problem is ARM's
arch_kmap_local_high_get(). This is where the lock is from.

> Yours,
> Linus Walleij

Sebastian