Re: [PATCH] sched: Add missing memory barrier in switch_mm_cid

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Mar 08, 2024 at 10:07:19AM -0500, Mathieu Desnoyers wrote:
> diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
> index 35389b2af88e..0d5e54201eb2 100644
> --- a/arch/x86/include/asm/barrier.h
> +++ b/arch/x86/include/asm/barrier.h
> @@ -79,6 +79,9 @@ do {									\
>  #define __smp_mb__before_atomic()	do { } while (0)
>  #define __smp_mb__after_atomic()	do { } while (0)
>  
> +/* Writing to CR3 provides a full memory barrier in switch_mm(). */
> +#define smp_mb__after_switch_mm()	do { } while (0)
> +
>  #include <asm-generic/barrier.h>
>  
>  /*
> diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
> index 961f4d88f9ef..5a6c94d7a598 100644
> --- a/include/asm-generic/barrier.h
> +++ b/include/asm-generic/barrier.h
> @@ -296,5 +296,13 @@ do {									\
>  #define io_stop_wc() do { } while (0)
>  #endif
>  
> +/*
> + * Architectures that guarantee an implicit smp_mb() in switch_mm()
> + * can override smp_mb__after_switch_mm.
> + */
> +#ifndef smp_mb__after_switch_mm
> +#define smp_mb__after_switch_mm()	smp_mb()
> +#endif
> +
>  #endif /* !__ASSEMBLY__ */
>  #endif /* __ASM_GENERIC_BARRIER_H */
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 2e5a95486a42..044d842c696c 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -79,6 +79,8 @@
>  # include <asm/paravirt_api_clock.h>
>  #endif
>  
> +#include <asm/barrier.h>
> +
>  #include "cpupri.h"
>  #include "cpudeadline.h"
>  
> @@ -3481,13 +3483,19 @@ static inline void switch_mm_cid(struct rq *rq,
>  		 * between rq->curr store and load of {prev,next}->mm->pcpu_cid[cpu].
>  		 * Provide it here.
>  		 */
> -		if (!prev->mm)                          // from kernel
> +		if (!prev->mm) {                        // from kernel
>  			smp_mb();
> -		/*
> -		 * user -> user transition guarantees a memory barrier through
> -		 * switch_mm() when current->mm changes. If current->mm is
> -		 * unchanged, no barrier is needed.
> -		 */
> +		} else {				// from user
> +			/*
> +			 * user -> user transition relies on an implicit
> +			 * memory barrier in switch_mm() when
> +			 * current->mm changes. If the architecture
> +			 * switch_mm() does not have an implicit memory
> +			 * barrier, it is emitted here.  If current->mm
> +			 * is unchanged, no barrier is needed.
> +			 */
> +			smp_mb__after_switch_mm();
> +		}

I'm fine with the change from the arm64 perspective but I guess we need
an ack from the x86 and sched maintainers. FWIW:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

-- 
Catalin