Re: [PATCH v2 1/2] locking/qrwlock: Fix bug in interrupt handling code

[Waiman Long]

On 06/11/2015 10:21 AM, Will Deacon wrote:
> Hi Waiman,
>
> On Tue, Jun 09, 2015 at 04:19:12PM +0100, Waiman Long wrote:
> > The qrwlock is fair in the process context, but becoming unfair when
> > in the interrupt context to support use cases like the tasklist_lock.
> > However, the unfair code in the interrupt context has problem that
> > may cause deadlock.
> >
> > The fast path increments the reader count. In the interrupt context,
> > the reader in the slowpath will wait until the writer release the
> > lock. However, if other readers have the lock and the writer is just
> > in the waiting mode. It will never get the write lock because the
> > that interrupt context reader has increment the count. This will
> > cause deadlock.
> I'm probably just being thick here, but I'm struggling to understand the
> deadlock case.
>
> If a reader enters the slowpath in interrupt context, we spin while
> (cnts&  _QW_WMASK) == _QW_LOCKED. Consequently, if there is a writer in
> the waiting state, that won't hold up the reader and so forward progress
> is ensured. When the reader unlocks, the reader count is decremented and
> the writer can take the lock.
>
> The only problematic case I can think of is if you had a steady stream of
> readers in interrupt context, but that doesn't seem likely (and I don't
> think this patch deals with that anyway).
>
> What am I missing?
>
> Will

You are right. It was my mistake. I misread my own code. I should have a 
comment to clarify that. I will send out a revised one next week.

Cheers,
Longman
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