Re: [RFC][PATCH 1/7] sched: Fix smp_call_function_single_async() usage for ILB

[Frederic Weisbecker]

On Tue, May 26, 2020 at 06:10:58PM +0200, Peter Zijlstra wrote:
> The recent commit: 90b5363acd47 ("sched: Clean up scheduler_ipi()")
> got smp_call_function_single_async() subtly wrong. Even though it will
> return -EBUSY when trying to re-use a csd, that condition is not
> atomic and still requires external serialization.
> 
> The change in kick_ilb() got this wrong.
> 
> While on first reading kick_ilb() has an atomic test-and-set that
> appears to serialize the use, the matching 'release' is not in the
> right place to actually guarantee this serialization.
> 
> Rework the nohz_idle_balance() trigger so that the release is in the
> IPI callback and thus guarantees the required serialization for the
> CSD.
> 
> Fixes: 90b5363acd47 ("sched: Clean up scheduler_ipi()")
> Reported-by: Qian Cai <cai@xxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Looks very good! Thanks!

Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>