On Mon, Sep 05, 2016 at 11:37:53AM +0200, Peter Zijlstra wrote: > So recently I've had two separate issues that touched upon > smp_mb__before_spinlock(). > > > Since its inception, our understanding of ACQUIRE, esp. as applied to > spinlocks, has changed somewhat. Also, I wonder if, with a simple > change, we cannot make it provide more. > > The problem with the comment is that the STORE done by spin_lock isn't > itself ordered by the ACQUIRE, and therefore a later LOAD can pass over > it and cross with any prior STORE, rendering the default WMB > insufficient (pointed out by Alan). > > Now, this is only really a problem on PowerPC and ARM64, the former of > which already defined smp_mb__before_spinlock() as a smp_mb(), the > latter does not, Will?
I just replied to that thread and, assuming I've groked the sched/core.c usage correctly, then it does look like we need to make that an smp_mb() with the current code. > The second issue I wondered about is spinlock transitivity. All except > powerpc have RCsc locks, and since Power already does a full mb, would > it not make sense to put it _after_ the spin_lock(), which would provide > the same guarantee, but also upgrades the section to RCsc. > > That would make all schedule() calls fully transitive against one > another. It would also match the way in which the arm64 atomic_*_return ops are implemented, since full barrier semantics are required there. > That is, would something like the below make sense? Works for me, but I'll do a fix to smp_mb__before_spinlock anyway for the stable tree. The only slight annoyance is that, on arm64 anyway, a store-release appearing in program order before the LOCK operation will be observed in order, so if the write of CONDITION=1 in the try_to_wake_up case used smp_store_release, we wouldn't need this barrier at all. Will
