On Thu, Jan 10, 2019 at 03:48:12PM +0100, Florian Westphal wrote: > Peter Zijlstra <[email protected]> wrote: > > /* > > * Ensure contents of newinfo are visible before assigning to > > * private. > > */ > > smp_wmb(); > > table->private = newinfo; > > > > we have: > > > > smp_store_release(&table->private, newinfo); > > > > But what store does that second smp_wmb() order against? The comment: > > > > /* make sure all cpus see new ->private value */ > > smp_wmb(); > > > > makes no sense what so ever, no smp_*() barrier can provide such > > guarantees. > > IIRC I added this at the request of a reviewer of an earlier iteration > of that patch. > > IIRC the concern was that compiler/hw could re-order > > smb_wmb(); > table->private = newinfo; > /* wait until all cpus are done with old table */ > > into: > > smb_wmb(); > /* wait until all cpus are done with old table */ > ... > table->private = newinfo; > > and that (obviously) makes the wait-loop useless.
The thing is, the 'wait for all cpus' thing is pure loads, not stores, so smp_wmb() is a complete NOP there. If you want to ensure those loads happen after that store (which does indeed seem like a sensible thing), you're going to have to use smp_mb().
