Re: [PATCH 1/5] locking: Introduce range reader/writer lock

[Davidlohr Bueso]

On Mon, 03 Apr 2017, Laurent Dufour wrote:

> Le Tue, 28 Mar 2017 09:39:18 -0700,
> Davidlohr Bueso <dave@xxxxxxxxxxxx> a écrit :
> > I'll wait to see if there are any more concerns and send a v2 with
> > your corrections.
>
> Hi Bavidlohr, I think there is a major issue regarding the task
> catching a signal in wait_for_range().
> I can see it when a thread is catching a signal, the process deadlock
> in exit path.
>
> Let's imagine all these tasks waiting for the complete range lock, so
> range doesn't matter:
>
> A get the lock in write
> B want the read lock => B->blocking_range=1 (because of A)
> C want the write lock => C->blocking_range=2 (A,B)
> D want the read lock => D->blocking_range=3 (A,B,C)
> => C catch a signal and exit wait_for_ranges()
> A release the lock
>        => B->blocking_range=0
>        => D->blocking_range=2 (D has not seen C removal)
> => B get the lock
> B release the lock
>        => D->blocking_range=1
>
> D remains blocked while no one has the lock !
>
> The issue is when removing a task from the interval tree, we
> should decrement all the blocking_ranges of the task added to that
> range after the one leaving... I can't see an easy fix for that :(
>
> Am I right ?

Yes. Peter had also mentioned the issue too. One way I though of fixing
the problem was to track the jiffies timestamp in a per range_rwlock
basis for when it was added, and in the signal_pending() case, along with
removing the lock from the tree, we iterate the tree again and decrement
the blocking_ranges for those with a higher timestamp. It would add some
overhead, but again this is the unlikely() case. It also adds an extra 8
bytes of footprint, but this is usually stack allocated.

Thanks,
Davidlohr