Re: [RFC][PATCH] sched: Fix TASK_DEAD race in finish_task_switch()

From: Linus Torvalds
Date: Tue Sep 29 2015 - 12:40:35 EST

On Tue, Sep 29, 2015 at 8:45 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> + *
> + * Pairs with the control dependency and rmb in try_to_wake_up().
> */

So this comment makes me nervous. A control dependency doesn't
actually do anything on powerpc and ARM (or alpha, or MIPS, or any
number of other architectures. Basically, a conditional branch ends up
not being the usual kind of data dependency (which works on everything
but alpha), because conditional branches are predicted and loads after
them are speculated.

Also, using smp_store_release() instead of a wmb() is going to be very
expensive on old ARM and a number of other not-so-great architectures.
On x86, both end up being just a scheduling thing. On other modern
architectures, store releases are fairly cheap, but wmb() is cheap

So long-term, the wmb->store_release conversion probably makes sense,
but it's at least debatable for now.

The one advantage that release/acquire do have is that particularly
when they pair up, they have much nicer semantics (ie you get the same
ordering guarantees as if you had a lock). But if they don't pair up,
there are actually some advantages to smp_wmb() over the alternatives.

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