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

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Date: Fri, 15 Dec 2023 13:52:14 +0800
From: "Aiqun Yu (Maria)" <quic_aiquny@...cinc.com>
To: "Eric W. Biederman" <ebiederm@...ssion.com>,
        Matthew Wilcox
	<willy@...radead.org>
CC: <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 12/14/2023 2:27 AM, Eric W. Biederman wrote:
> 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
Any better ideas and suggestions are welcomed. :)
>> so readers don't know that there's a pending writer.  Also, I've see >> 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.
> 
Let me think about this.
It seems a possible because there is a special behavior from reader side 
when in interrupt it will directly get the lock regardless of the 
pending writer.

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

-- 
Thx and BRs,
Aiqun(Maria) Yu