Messages in this thread | | | Date | Thu, 22 Dec 2016 15:00:10 +0100 | From | Peter Zijlstra <> | Subject | Re: Perf hotplug lockup in v4.9-rc8 |
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On Thu, Dec 22, 2016 at 09:45:09AM +0100, Peter Zijlstra wrote: > On Mon, Dec 12, 2016 at 01:42:28PM +0100, Peter Zijlstra wrote:
> > > What are you trying to order here? > > > > I suppose something like this: > > > > > > CPU0 CPU1 CPU2 > > > > (current == t) > > > > t->perf_event_ctxp[] = ctx; > > smp_mb(); > > cpu = task_cpu(t); > > > > switch(t, n); > > migrate(t, 2); > > switch(p, t); > > > > ctx = t->perf_event_ctxp[]; // must not be NULL > > > > So I think I can cast the above into a test like: > > W[x] = 1 W[y] = 1 R[z] = 1 > mb mb mb > R[y] = 0 W[z] = 1 R[x] = 0 > > Where x is the perf_event_ctxp[], y is our task's cpu and z is our task > being placed on the rq of cpu2. > > See also commit: 8643cda549ca ("sched/core, locking: Document > Program-Order guarantees"), Independent of which cpu initiates the > migration between CPU1 and CPU2 there is ordering between the CPUs.
I think that when we assume RCpc locks, the above CPU1 mb ends up being something like an smp_wmb() (ie. non transitive). CPU2 needs to do a context switch between observing the task on its runqueue and getting to switching in perf-events for the task, which keeps that a full mb.
Now, if only this model would have locks in ;-)
> This would then translate into something like: > > C C-peterz > > { > } > > P0(int *x, int *y) > { > int r1; > > WRITE_ONCE(*x, 1); > smp_mb(); > r1 = READ_ONCE(*y); > } > > P1(int *y, int *z) > { > WRITE_ONCE(*y, 1); > smp_mb();
And this modified to: smp_wmb()
> WRITE_ONCE(*z, 1); > } > > P2(int *x, int *z) > { > int r1; > int r2; > > r1 = READ_ONCE(*z); > smp_mb(); > r2 = READ_ONCE(*x); > } > > exists > (0:r1=0 /\ 2:r1=1 /\ 2:r2=0)
Still results in the same outcome.
If however we change P2's barrier into a smp_rmb() it does become possible, but as said above, there's a context switch in between which implies a full barrier so no worries.
Similar if I replace everything z with smp_store_release() and smp_load_acquire().
Of course, its entirely possible the litmus test doesn't reflect reality, I still find it somewhat hard to write these things.
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