Re: [PATCH] arm64/mm: Add memory barrier for mm_cid

[Will Deacon]

On Tue, Mar 05, 2024 at 02:53:35PM +0000, levi.yun wrote:
> Currently arm64's switch_mm() doesn't always have an smp_mb()
> which the core scheduler code has depended upon since commit:
> 
>     commit 223baf9d17f25 ("sched: Fix performance regression introduced by mm_cid")
> 
> If switch_mm() doesn't call smp_mb(), sched_mm_cid_remote_clear()
> can unset the activly used cid when it fails to observe active task after it
> sets lazy_put.
> 
> By adding an smp_mb() in arm64's check_and_switch_context(),
> Guarantee to observe active task after sched_mm_cid_remote_clear()
> success to set lazy_put.
> 
> Signed-off-by: levi.yun <yeoreum.yun@xxxxxxx>
> Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced by mm_cid")
> Cc: <stable@xxxxxxxxxxxxxxx> # 6.4.x
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Aaron Lu <aaron.lu@xxxxxxxxx>
> ---
>  I'm really sorry if you got this multiple times.
>  I had some problems with the SMTP server...
> 
>  arch/arm64/mm/context.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/arm64/mm/context.c b/arch/arm64/mm/context.c
> index 188197590fc9..7a9e8e6647a0 100644
> --- a/arch/arm64/mm/context.c
> +++ b/arch/arm64/mm/context.c
> @@ -268,6 +268,11 @@ void check_and_switch_context(struct mm_struct *mm)
>  	 */
>  	if (!system_uses_ttbr0_pan())
>  		cpu_switch_mm(mm->pgd, mm);
> +
> +	/*
> +	 * See the comments on switch_mm_cid describing user -> user transition.
> +	 */
> +	smp_mb();
>  }

We already have a stronger barrier than smp_mb() (dsb ish) in __switch_to().
Is that not sufficient?

Will