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

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Message-ID: <87o7eu7ybq.fsf@email.froward.int.ebiederm.org>
Date:   Wed, 13 Dec 2023 12:27:05 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Maria Yu <quic_aiquny@...cinc.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

Matthew Wilcox <willy@...radead.org> writes:

> 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.

Yes.  It seems to make sense to distinguish between write_lock_irq and
write_lock_irqsave and fix this for all of write_lock_irq.

Either that or someone can put in the work to start making the
tasklist_lock go away.

Eric