Messages in this thread | | | Date | Mon, 21 Jan 2019 13:25:26 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched: Use READ_ONCE()/WRITE_ONCE() in task_cpu()/__set_task_cpu() |
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On Mon, Jan 21, 2019 at 11:51:21AM +0100, Andrea Parri wrote: > On Wed, Jan 16, 2019 at 07:42:18PM +0100, Andrea Parri wrote: > > The smp_wmb() in move_queued_task() (c.f., __set_task_cpu()) pairs with > > the composition of the dependency and the ACQUIRE in task_rq_lock(): > > > > move_queued_task() task_rq_lock() > > > > [S] ->on_rq = MIGRATING [L] rq = task_rq() > > WMB (__set_task_cpu()) ACQUIRE (rq->lock); > > [S] ->cpu = new_cpu [L] ->on_rq > > > > where "[L] rq = task_rq()" is ordered before "ACQUIRE (rq->lock)" by an > > address dependency and, in turn, "ACQUIRE (rq->lock)" is ordered before > > "[L] ->on_rq" by the ACQUIRE itself. > > > > Use READ_ONCE() to load ->cpu in task_rq() (c.f., task_cpu()) to honour > > this address dependency between loads; also, mark the store to ->cpu in > > __set_task_cpu() by using WRITE_ONCE() in order to tell the compiler to > > not mess/tear this (synchronizing) memory access. > > In the light of the recent discussion about the integration of plain > accesses in the LKMM (c.f., e.g., [1] and discussion thereof), I was > considering even further changes to this in order to "reinforce" the > above smp_wmb(). Here's two approaches (one of): > > 1) replace this smp_wmb()+WRITE_ONCE() with an smp_store_release(); > > 2) or keep this smp_wmb()+WRITE_ONCE(), but use {WRITE,READ}_ONCE() > also for the accesses to ->on_rq.
That should be the least painful I think. Note that we never store a value larger than a single byte in that word, so tearing shouldn't be a problem, but yes, that makes it all neat and tidy.
> What do you think? (maybe I'm just being too paranoid?) > > Adding Will to the Cc: ((1) should be "painless" for x86, not sure > about arm64...)
ARM64 should be fine, it is 32bit ARM that will suffer, because it uses smp_mb() to implement acquire/release.
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