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SubjectRe: [PATCH 5/6] kernel/locking: Prevent slowpath writers getting held up by fastpath
HI Peter,

Thanks for having a look.

On Thu, Oct 05, 2017 at 03:56:18PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 05, 2017 at 01:54:56PM +0100, Will Deacon wrote:
> > When a prospective writer takes the qrwlock locking slowpath due to the
> > lock being held, it attempts to cmpxchg the wmode field from 0 to
> > _QW_WAITING so that concurrent lockers also take the slowpath and queue
> > on the spinlock accordingly, allowing the lockers to drain.
> >
> > Unfortunately, this isn't fair, because a fastpath writer that comes in
> > after the lock is made available but before the _QW_WAITING flag is set
> > can effectively jump the queue. If there is a steady stream of prospective
> > writers, then the waiter will be held off indefinitely.
> >
> > This patch restores fairness by separating _QW_WAITING and _QW_LOCKED
> > into two bits in the wmode byte and having the waiter set _QW_WAITING
> > unconditionally. This then forces the slow-path for concurrent lockers,
> > but requires that a writer unlock operation performs an
> > atomic_sub_release instead of a store_release so that the waiting status
> > is preserved.
>
> > diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
> > index 02c0a768e6b0..8b7edef500e5 100644
> > --- a/include/asm-generic/qrwlock.h
> > +++ b/include/asm-generic/qrwlock.h
> > @@ -41,7 +41,7 @@
> > * +----+----+----+----+
> > */
> > #define _QW_WAITING 1 /* A writer is waiting */
> > -#define _QW_LOCKED 0xff /* A writer holds the lock */
> > +#define _QW_LOCKED 2 /* A writer holds the lock */
> > #define _QW_WMASK 0xff /* Writer mask */
> > #define _QR_SHIFT 8 /* Reader count shift */
> > #define _QR_BIAS (1U << _QR_SHIFT)
> > @@ -134,7 +134,7 @@ static inline void queued_read_unlock(struct qrwlock *lock)
> > */
> > static inline void queued_write_unlock(struct qrwlock *lock)
> > {
> > - smp_store_release(&lock->wmode, 0);
> > + (void)atomic_sub_return_release(_QW_LOCKED, &lock->cnts);
> > }
>
> That is a fairly painful hit on x86. Changes a regular store into an
> "LOCK XADD" +20 cycles right there.

Yeah, I mentioned that in the cover letter which is also why it's at the end
of the series ;) However, it's worth noting that this is the same as the
reader unlock path and, as it stands, there's a real risk of writer
starvation with the current code which isn't great for a queued lock.

> Can't we steal one of the reader bits for waiting?

I considered this at LPC and somehow convinced myself it didn't work, but
actually all it's really doing is making the _QW_LOCKED bit a byte, so it
should work fine.

I'll work that into v2.

Will

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