linux-kernel - RE: [RFC PATCH v2 2/4] Documentation: RCU: Refer to ptr

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Message-ID: <72af935f4a2a4e23b68845d2f6855103@AcuMS.aculab.com>
Date: Sun, 6 Oct 2024 19:52:49 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Mathieu Desnoyers' <mathieu.desnoyers@...icios.com>, Boqun Feng
	<boqun.feng@...il.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Linus
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Subject: RE: [RFC PATCH v2 2/4] Documentation: RCU: Refer to ptr_eq()

From: Mathieu Desnoyers
> Sent: 04 October 2024 19:28
> 
> Refer to ptr_eq() in the rcu_dereference() documentation.
> 
> ptr_eq() is a mechanism that preserves address dependencies when
> comparing pointers, and should be favored when comparing a pointer
> obtained from rcu_dereference() against another pointer.

Why does this ever really matter for rcu?

The check just ensure that any speculative load uses
a specific one of the pointers when they are different.
This can only matter if you care about the side effects
of the speculative load.

But rcu is all about (things like) lockless list following.
So you need to wait until it is impossible for another
execution context to have a reference to some memory
before actually completely invalidating it (ie kfree()).

And that 50 line comment is pointless.

	David

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