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

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Date: Wed, 3 Jan 2024 18:18:07 +0000
From: Matthew Wilcox <willy@...radead.org>
To: "Aiqun Yu (Maria)" <quic_aiquny@...cinc.com>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>,
	Hillf Danton <hdanton@...a.com>, kernel@...cinc.com,
	quic_pkondeti@...cinc.com, keescook@...omium.org,
	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 Wed, Jan 03, 2024 at 10:58:33AM +0800, Aiqun Yu (Maria) wrote:
> On 1/2/2024 5:14 PM, Matthew Wilcox wrote:
> > > > -void __lockfunc queued_write_lock_slowpath(struct qrwlock *lock)
> > > > +void __lockfunc queued_write_lock_slowpath(struct qrwlock *lock, bool irq)
> > > >    {
> > > >    	int cnts;
> > > > @@ -82,7 +83,11 @@ void __lockfunc queued_write_lock_slowpath(struct qrwlock *lock)
> > > Also a new state showed up after the current design:
> > > 1. locked flag with _QW_WAITING, while irq enabled.
> > > 2. And this state will be only in interrupt context.
> > > 3. lock->wait_lock is hold by the write waiter.
> > > So per my understanding, a different behavior also needed to be done in
> > > queued_write_lock_slowpath:
> > >    when (unlikely(in_interrupt())) , get the lock directly.
> > 
> > I don't think so.  Remember that write_lock_irq() can only be called in
> > process context, and when interrupts are enabled.
> In current kernel drivers, I can see same lock called with write_lock_irq
> and write_lock_irqsave in different drivers.
> 
> And this is the scenario I am talking about:
> 1. cpu0 have task run and called write_lock_irq.(Not in interrupt context)
> 2. cpu0 hold the lock->wait_lock and re-enabled the interrupt.

Oh, I missed that it was holding the wait_lock.  Yes, we also need to
release the wait_lock before spinning with interrupts disabled.

> I was thinking to support both write_lock_irq and write_lock_irqsave with
> interrupt enabled together in queued_write_lock_slowpath.
> 
> That's why I am suggesting in write_lock_irqsave when (in_interrupt()),
> instead spin for the lock->wait_lock, spin to get the lock->cnts directly.

Mmm, but the interrupt could come in on a different CPU and that would
lead to it stealing the wait_lock from the CPU which is merely waiting
for the readers to go away.