linux-kernel - Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for tasklist

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Message-ID: <ZXnaNSrtaWbS2ivU@casper.infradead.org>
Date:   Wed, 13 Dec 2023 16:22:13 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Maria Yu <quic_aiquny@...cinc.com>
Cc:     ebiederm@...ssion.com, kernel@...cinc.com,
        quic_pkondeti@...cinc.com, keescook@...omium.or,
        viro@...iv.linux.org.uk, brauner@...nel.org, oleg@...hat.com,
        dhowells@...hat.com, jarkko@...nel.org, paul@...l-moore.com,
        jmorris@...ei.org, serge@...lyn.com, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        keyrings@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for
 tasklist_lock

On Wed, Dec 13, 2023 at 06:17:45PM +0800, Maria Yu wrote:
> +static inline void write_lock_tasklist_lock(void)
> +{
> +	while (1) {
> +		local_irq_disable();
> +		if (write_trylock(&tasklist_lock))
> +			break;
> +		local_irq_enable();
> +		cpu_relax();

This is a bad implementation though.  You don't set the _QW_WAITING flag
so readers don't know that there's a pending writer.  Also, I've seen
cpu_relax() pessimise CPU behaviour; putting it into a low-power mode
that takes a while to wake up from.

I think the right way to fix this is to pass a boolean flag to
queued_write_lock_slowpath() to let it know whether it can re-enable
interrupts while checking whether _QW_WAITING is set.