From: Will Deacon <will.deacon@arm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Paul McKenney" <paulmck@linux.vnet.ibm.com>,
"Alan Stern" <stern@rowland.harvard.edu>,
"Andrea Parri" <parri.andrea@gmail.com>,
"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
priyalee.kushwaha@intel.com,
"Stanisław Drozd" <drozdziak1@gmail.com>,
"Arnd Bergmann" <arnd@arndb.de>,
ldr709@gmail.com, "Thomas Gleixner" <tglx@linutronix.de>,
"Peter Zijlstra" <peterz@infradead.org>,
"Josh Triplett" <josh@joshtriplett.org>,
"Nicolas Pitre" <nico@linaro.org>,
"Krister Johansen" <kjlx@templeofstupid.com>,
"Vegard Nossum" <vegard.nossum@oracle.com>,
dcb314@hotmail.com, "Wu Fengguang" <fengguang.wu@intel.com>,
"Frederic Weisbecker" <fweisbec@gmail.com>,
"Rik van Riel" <riel@redhat.com>,
"Steven Rostedt" <rostedt@goodmis.org>,
"Ingo Molnar" <mingo@kernel.org>,
"Luc Maranget" <luc.maranget@inria.fr>,
"Jade Alglave" <j.alglave@ucl.ac.uk>
Subject: Re: [GIT PULL rcu/next] RCU commits for 4.13
Date: Thu, 29 Jun 2017 12:38:48 +0100 [thread overview]
Message-ID: <20170629113848.GA18630@arm.com> (raw)
In-Reply-To: <CA+55aFwLq5oPvY5HwpKq9LkCuQ5No7O5g=+ij1T68ONu-gOm_Q@mail.gmail.com>
[turns out I've not been on cc for this thread, but Jade pointed me to it
and I see my name came up at some point!]
On Wed, Jun 28, 2017 at 05:05:46PM -0700, Linus Torvalds wrote:
> On Wed, Jun 28, 2017 at 4:54 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> >
> > Linus, are you dead-set against defining spin_unlock_wait() to be
> > spin_lock + spin_unlock? For example, is the current x86 implementation
> > of spin_unlock_wait() really a non-negotiable hard requirement? Or
> > would you be willing to live with the spin_lock + spin_unlock semantics?
>
> So I think the "same as spin_lock + spin_unlock" semantics are kind of insane.
>
> One of the issues is that the same as "spin_lock + spin_unlock" is
> basically now architecture-dependent. Is it really the
> architecture-dependent ordering you want to define this as?
>
> So I just think it's a *bad* definition. If somebody wants something
> that is exactly equivalent to spin_lock+spin_unlock, then dammit, just
> do *THAT*. It's completely pointless to me to define
> spin_unlock_wait() in those terms.
>
> And if it's not equivalent to the *architecture* behavior of
> spin_lock+spin_unlock, then I think it should be descibed in terms
> that aren't about the architecture implementation (so you shouldn't
> describe it as "spin_lock+spin_unlock", you should describe it in
> terms of memory barrier semantics.
>
> And if we really have to use the spin_lock+spinunlock semantics for
> this, then what is the advantage of spin_unlock_wait at all, if it
> doesn't fundamentally avoid some locking overhead of just taking the
> spinlock in the first place?
Just on this point -- the arm64 code provides the same ordering semantics
as you would get from a lock;unlock sequence, but we can optimise that
when compared to an actual lock;unlock sequence because we don't need to
wait in turn for our ticket. I suspect something similar could be done
if/when we move to qspinlocks.
Whether or not this is actually worth optimising is another question, but
it is worth noting that unlock_wait can be implemented more cheaply than
lock;unlock, whilst providing the same ordering guarantees (if that's
really what we want -- see my reply to Paul).
Simplicity tends to be my preference, so ripping this out would suit me
best ;)
Will
next prev parent reply other threads:[~2017-06-29 11:38 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-12 21:37 [GIT PULL rcu/next] RCU commits for 4.13 Paul E. McKenney
2017-06-13 6:41 ` Ingo Molnar
2017-06-14 2:54 ` Andrea Parri
2017-06-14 4:33 ` Paul E. McKenney
2017-06-14 14:33 ` Andrea Parri
2017-06-14 20:23 ` Paul E. McKenney
2017-06-19 16:24 ` Andrea Parri
2017-06-27 20:58 ` Paul E. McKenney
2017-06-27 21:48 ` Linus Torvalds
2017-06-27 23:37 ` Paul E. McKenney
2017-06-28 15:31 ` Alan Stern
2017-06-28 17:03 ` Paul E. McKenney
2017-06-28 20:16 ` Alan Stern
2017-06-28 23:54 ` Paul E. McKenney
2017-06-29 0:05 ` Linus Torvalds
2017-06-29 0:45 ` Paul E. McKenney
2017-06-29 3:17 ` Boqun Feng
2017-06-29 18:47 ` Paul E. McKenney
2017-06-29 11:36 ` Will Deacon
2017-06-29 11:38 ` Will Deacon [this message]
2017-06-29 15:59 ` Alan Stern
2017-06-29 18:11 ` Paul E. McKenney
2017-06-30 2:51 ` Boqun Feng
2017-06-30 4:02 ` Paul E. McKenney
2017-06-30 5:16 ` Boqun Feng
2017-06-30 17:31 ` Paul E. McKenney
2017-06-29 18:46 ` Paul E. McKenney
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