Re: write_lock_irq(&tasklist_lock)

[Peter Zijlstra]

On Tue, May 22, 2018 at 02:31:42PM -0700, Linus Torvalds wrote:
> On Tue, May 22, 2018 at 2:17 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > qrwlock is a fair lock and should not exhibit writer starvation.
> 
> We actually have a special rule to make it *not* be fair, in that
> interrupts are allowed to take the read lock if there are readers - even if
> there are waiting writers.

Urgh, right.. would be interesting to know how much of that is happening
in that workload. I assumed the readers were mostly due to the syscalls
the reporter talked about, and those should not trigger that case.

> > You basically want to spin-wait with interrupts enabled, right?
> 
> That was the intent of my (untested) pseudo-code. It should work fine. Note
> that I used write_trylock() only, so there is no queueing (which also
> implies no fairness).
> 
> I'm not saying it's a _good_ idea.  I'm saying it might work if all you
> worry about is the irq-disabled part.

Right, if you make it unfair and utterly prone to starvation then yes,
you can make it 'work'.