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Message-ID: <6e762e8e-b031-4e37-97c1-56390c9b8076@quicinc.com>
Date: Wed, 27 Dec 2023 09:41:29 +0800
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Subject: Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for
 tasklist_lock
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To: Hillf Danton <hdanton@sina.com>, Matthew Wilcox <willy@infradead.org>
CC: "Eric W. Biederman" <ebiederm@xmission.com>,
        <linux-kernel@vger.kernel.org>
References: <20231213101745.4526-1-quic_aiquny@quicinc.com>
 <ZXnaNSrtaWbS2ivU@casper.infradead.org>
 <20231226104652.1491-1-hdanton@sina.com>
From: "Aiqun Yu (Maria)" <quic_aiquny@quicinc.com>
In-Reply-To: <20231226104652.1491-1-hdanton@sina.com>
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On 12/26/2023 6:46 PM, Hillf Danton wrote:
> On Wed, 13 Dec 2023 12:27:05 -0600 Eric W. Biederman <ebiederm@xmission.com>
>> 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.
> 
> 	lock(&lock->wait_lock)
> 	enable irq
> 	int
> 	lock(&lock->wait_lock)
> 
> You are adding chance for recursive locking.

Thx for the comments for discuss of the deadlock possibility. While I 
think deadlock can be differentiate with below 2 scenarios:
1. queued_write_lock_slowpath being triggered in interrupt context.
   tasklist_lock don't have write_lock_irq(save) in interrupt context.
   while for common rw lock, maybe write_lock_irq(save) usage in 
interrupt context is a possible.
   so may introduce a state when lock->wait_lock is released and left 
the _QW_WAITING flag.
Welcome others to suggest on designs and comments.

2.queued_read_lock_slowpath can be triggered in interrupt context. And 
it already have the handle to avoid possible deadlock.
In the queued_read_lock_slowpath, there is check whether current context 
is in interrupt or not, and get the lock directly of only write lock 
waiting.

Pls reference[1]:
	/*
	 * Readers come here when they cannot get the lock without waiting
	 */
	if (unlikely(in_interrupt())) {
		/*
		 * Readers in interrupt context will get the lock immediately
		 * if the writer is just waiting (not holding the lock yet),
		 * so spin with ACQUIRE semantics until the lock is available
		 * without waiting in the queue.
		 */
		atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
		return;
	}

[1]: 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/locking/qrwlock.c
>>
>> 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

-- 
Thx and BRs,
Aiqun(Maria) Yu