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

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Date: Tue, 26 Dec 2023 18:46:52 +0800
From: Hillf Danton <hdanton@...a.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>,
	Maria Yu <quic_aiquny@...cinc.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for tasklist_lock

On Wed, 13 Dec 2023 12:27:05 -0600 Eric W. Biederman <ebiederm@...ssion.com>
> 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.

	lock(&lock->wait_lock)
	enable irq
	int
	lock(&lock->wait_lock)

You are adding chance for recursive locking.
> 
> 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