Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.33 as permitted sender) client-ip=23.128.96.33;
From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Maria Yu <quic_aiquny@quicinc.com>, kernel@quicinc.com,
        quic_pkondeti@quicinc.com, keescook@chromium.or,
        viro@zeniv.linux.org.uk, brauner@kernel.org, oleg@redhat.com,
        dhowells@redhat.com, jarkko@kernel.org, paul@paul-moore.com,
        jmorris@namei.org, serge@hallyn.com, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        keyrings@vger.kernel.org, linux-security-module@vger.kernel.org,
        linux-arm-msm@vger.kernel.org
References: <20231213101745.4526-1-quic_aiquny@quicinc.com>
        <ZXnaNSrtaWbS2ivU@casper.infradead.org>
Date:   Wed, 13 Dec 2023 12:27:05 -0600
In-Reply-To: <ZXnaNSrtaWbS2ivU@casper.infradead.org> (Matthew Wilcox's message
        of "Wed, 13 Dec 2023 16:22:13 +0000")
Message-ID: <87o7eu7ybq.fsf@email.froward.int.ebiederm.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for
 tasklist_lock
Precedence: bulk

Matthew Wilcox <willy@infradead.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