On Thu, Jul 24, 2014 at 11:18:16AM -0700, Linus Torvalds wrote:

On Thu, Jul 24, 2014 at 5:58 AM, Peter Zijlstra [off-list ref] wrote:

quoted

So going by the nifty picture rostedt made:

[   61.454336]        CPU0                    CPU1
[   61.454336]        ----                    ----
[   61.454336]   lock(&(&p->alloc_lock)->rlock);
[   61.454336]                                local_irq_disable();
[   61.454336]                                lock(tasklist_lock);
[   61.454336]                                lock(&(&p->alloc_lock)->rlock);
[   61.454336]   <Interrupt>
[   61.454336]     lock(tasklist_lock);

So this *should* be fine. It always has been in the past, and it was
certainly the *intention* that it should continue to work with
qrwlock, even in the presense of pending writers on other cpu's.

The qrwlock rules are that a read-lock in an interrupt is still going
to be unfair and succeed if there are other readers.

Ah, indeed. Should have checked :/

So it sounds to me like the new lockdep rules in tip/master are too
strict and are throwing a false positive.

Right. Waiman can you have a look?