Messages in this thread | | | Date | Thu, 5 Oct 2017 15:42:57 +0100 | From | Will Deacon <> | Subject | Re: [PATCH 5/6] kernel/locking: Prevent slowpath writers getting held up by fastpath |
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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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