From: Darren Hart <dvhart@linux.intel.com>
To: Davidlohr Bueso <davidlohr@hp.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
peterz@infradead.org, tglx@linutronix.de,
paulmck@linux.vnet.ibm.com, efault@gmx.de, jeffm@suse.com,
torvalds@linux-foundation.org, jason.low2@hp.com,
Waiman.Long@hp.com, tom.vaden@hp.com, scott.norton@hp.com,
aswin@hp.com
Subject: Re: [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup
Date: Tue, 07 Jan 2014 09:40:50 -0800 [thread overview]
Message-ID: <1389116450.5785.16.camel@dvhart-mobl4.amr.corp.intel.com> (raw)
In-Reply-To: <1389065371.9937.15.camel@buesod1.americas.hpqcorp.net>
On Mon, 2014-01-06 at 19:29 -0800, Davidlohr Bueso wrote:
> On Mon, 2014-01-06 at 12:52 -0800, Darren Hart wrote:
> > On Thu, 2014-01-02 at 07:05 -0800, Davidlohr Bueso wrote:
> > I thought someone, Peter Z?, had commented on these CONFIG_SMP bits. Are
> > they really necessary? Does smp_mb__after_atomic_inc() and smp_rmb() not
> > already just do the right thing as far as we're concerned here?
>
> I don't think so. Thomas and I agreed that this was in fact the way to
> go. I rechecked old email and didn't notice any objections to
> CONFIG_SMP. Also for things like hb_waiters_pending we definitely need
> it.
I'll happily defer to Thomas here.
> >
> > Given the subtlety of the implementation - I think it would be good to
> > explicitly annotate the get_futex_key() call site in futex_wake() as
> > providing the MB (B).
> >
> > Similar comment for futex_wait() and futex_requeue() for MB (A).
> >
> > These will also raise the appropriate red flags for people looking to
> > optimize or modify these paths in the future. It would be good to have
> > it in the top level futex_* function to make the MB placement and
> > relationship explicitly clear.
> >
>
> Something quite similar was already there for v2 but PeterZ's feedback
> made me update the main documentation at the top of futex.c to as it is
> now...
I don't want to block this any longer - but as complicated and
non-obvious as this is, I would *MUCH* prefer we document the memory
barrier point in the top level algorithm. If Peter/Thomas/Linus/Ingo
object, so be it, but otherwise let's err on the side of overly explicit
documentation.
Peter/Thomas/Linus/Ingo: Do any of you object to adding the three memory
barrier comments to the high level functions? futex_wait, futex_wake,
futex_requeue?
--
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel
next prev parent reply other threads:[~2014-01-07 17:41 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-01-02 15:05 [PATCH v5 0/4] futex: Wakeup optimizations Davidlohr Bueso
2014-01-02 15:05 ` [PATCH v5 1/4] futex: Misc cleanups Davidlohr Bueso
2014-01-11 6:43 ` Paul E. McKenney
2014-01-02 15:05 ` [PATCH v5 2/4] futex: Larger hash table Davidlohr Bueso
2014-01-11 7:37 ` Paul E. McKenney
2014-01-02 15:05 ` [PATCH v5 3/4] futex: Document ordering guarantees Davidlohr Bueso
2014-01-06 18:58 ` Darren Hart
2014-01-11 7:40 ` Paul E. McKenney
2014-01-02 15:05 ` [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup Davidlohr Bueso
2014-01-02 19:23 ` Linus Torvalds
2014-01-02 20:59 ` Davidlohr Bueso
2014-01-06 20:56 ` Darren Hart
2014-01-06 20:52 ` Darren Hart
2014-01-07 3:29 ` Davidlohr Bueso
2014-01-07 17:40 ` Darren Hart [this message]
2014-01-11 9:49 ` Paul E. McKenney
2014-01-11 9:52 ` Paul E. McKenney
2014-01-11 18:21 ` Davidlohr Bueso
2014-01-06 0:59 ` [PATCH v5 0/4] futex: Wakeup optimizations Davidlohr Bueso
2014-01-06 1:38 ` [PATCH 5/4] futex: silence uninitialized warnings Davidlohr Bueso
2014-01-06 18:48 ` Darren Hart
2014-01-07 2:55 ` Linus Torvalds
2014-01-07 3:02 ` Davidlohr Bueso
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