linux-kernel - Re: [RFC PATCH 1/1] fs/namespace.c: use spinlock instead of busy loop

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Message-ID: <20200617112719.GI2531@hirez.programming.kicks-ass.net>
Date:   Wed, 17 Jun 2020 13:27:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] fs/namespace.c: use spinlock instead of busy loop

On Wed, Jun 17, 2020 at 12:46:58PM +0206, John Ogness wrote:
> The MNT_WRITE_HOLD flag is used to manually implement a per-cpu
> optimized rwsem using busy waiting. This implementation is a problem
> on PREEMPT_RT because write_seqlock() on @mount_lock (i.e. taking a
> spinlock) does not disable preemption. This allows a writer to
> preempt a task that has set MNT_WRITE_HOLD and thus that writer will
> live lock in __mnt_want_write() due to busy looping with preemption
> disabled.
> 

Why can't you use a regular percpu-rwsem for this?