Re: [RFC] arm64: Enforce observed order for spinlock and data
From: Peter Zijlstra
Date: Fri Sep 30 2016 - 14:52:57 EST
On Fri, Sep 30, 2016 at 01:40:57PM -0400, Brent DeGraaf wrote:
> Prior spinlock code solely used load-acquire and store-release
> semantics to ensure ordering of the spinlock lock and the area it
> protects. However, store-release semantics and ordinary stores do
> not protect against accesses to the protected area being observed
> prior to the access that locks the lock itself.
> While the load-acquire and store-release ordering is sufficient
> when the spinlock routines themselves are strictly used, other
> kernel code that references the lock values directly (e.g. lockrefs)
> could observe changes to the area protected by the spinlock prior
> to observance of the lock itself being in a locked state, despite
> the fact that the spinlock logic itself is correct.
> Barriers were added to all the locking routines wherever necessary
> to ensure that outside observers which read the lock values directly
> will not observe changes to the protected data before the lock itself
> is observed.
I cannot see this going in. You're making spinlocks far more expensive
in the common case that doesn't need this.
Please enumerate the special cases (there's more than just lockref?) and