linux-kernel - Re: [RFC 1/2] rwsem: introduce upgrade

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Message-ID: <ZxAAgikXWswSJ76D@casper.infradead.org>
Date: Wed, 16 Oct 2024 19:05:54 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Waiman Long <llong@...hat.com>
Cc: lizhe.67@...edance.com, peterz@...radead.org, mingo@...hat.com,
	will@...nel.org, boqun.feng@...il.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC 1/2] rwsem: introduce upgrade_read interface

On Wed, Oct 16, 2024 at 10:23:14AM -0400, Waiman Long wrote:
> Another alternative that I have been thinking about is a down_read() variant
> with intention to upgrade later. This will ensure that only one active
> reader is allowed to upgrade later. With this, upgrade_read() will always
> succeed, maybe with some sleeping, as long as the correct down_read() is
> used.

How is that different from Kent's SIX locks other than you can take an
rwsem for write immediately (SIX locks have to be taken for Intent and
then Upgraded)?