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.
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
