Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for tasklist_lock

[Aiqun Yu (Maria)]

On 12/26/2023 6:46 PM, Hillf Danton wrote:
> On Wed, 13 Dec 2023 12:27:05 -0600 Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> > Matthew Wilcox <willy@xxxxxxxxxxxxx> 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