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

[Matthew Wilcox]

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.