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Message-ID: <75de85ae-ebcb-4f86-8cbf-749708cb3668@quicinc.com>
Date: Fri, 15 Dec 2023 13:52:14 +0800
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Subject: Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for
 tasklist_lock
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To: "Eric W. Biederman" <ebiederm@xmission.com>,
        Matthew Wilcox
	<willy@infradead.org>
CC: <kernel@quicinc.com>, <quic_pkondeti@quicinc.com>, <keescook@chromium.or>,
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References: <20231213101745.4526-1-quic_aiquny@quicinc.com>
 <ZXnaNSrtaWbS2ivU@casper.infradead.org>
 <87o7eu7ybq.fsf@email.froward.int.ebiederm.org>
From: "Aiqun Yu (Maria)" <quic_aiquny@quicinc.com>
In-Reply-To: <87o7eu7ybq.fsf@email.froward.int.ebiederm.org>
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On 12/14/2023 2:27 AM, Eric W. Biederman wrote:
> 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
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