linux-kernel - Re: [RFC 0/2] rwsem: introduce upgrade

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Message-ID: <20241016080955.GR16066@noisy.programming.kicks-ass.net>
Date: Wed, 16 Oct 2024 10:09:55 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: lizhe.67@...edance.com
Cc: mingo@...hat.com, will@...nel.org, longman@...hat.com,
	boqun.feng@...il.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC 0/2] rwsem: introduce upgrade_read interface

On Wed, Oct 16, 2024 at 12:35:58PM +0800, lizhe.67@...edance.com wrote:
> From: Li Zhe <lizhe.67@...edance.com>
> 
> In the current kernel rwsem implementation, there is an interface to
> downgrade write lock to read lock, but there is no interface to upgrade
> a read lock to write lock. This means that in order to acquire write
> lock while holding read lock, we have to release the read lock first and
> then acquire the write lock, which will introduce some troubles in
> concurrent programming. This patch set provides the 'upgrade_read' interface
> to solve this problem. This interface can change a read lock to a write
> lock.

upgrade-read is fundamentally prone to deadlocks. Imagine two concurrent
invocations, each waiting for all readers to go away before proceeding
to upgrade to a writer.

Any solution to fixing that will end up being semantically similar to
dropping the read lock and acquiring a write lock -- there will not be a
single continuous critical section.

As such, this interface makes no sense.