Messages in this thread | | | Date | Thu, 29 Jun 2017 12:38:48 +0100 | From | Will Deacon <> | Subject | Re: [GIT PULL rcu/next] RCU commits for 4.13 |
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[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
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