Subject: Re: [PATCH 4/4] powerpc/64s/radix: Fix mm_cpumask trimming race vs kthread_use_mm

From: Michael Ellerman <[email-masked]>
Date: Tue, 01 Sep 2020 22:00:20 +1000

Nicholas Piggin <[email-masked]> writes:
> Commit 0cef77c7798a7 ("powerpc/64s/radix: flush remote CPUs out of
> single-threaded mm_cpumask") added a mechanism to trim the mm_cpumask of
> a process under certain conditions. One of the assumptions is that
> mm_users would not be incremented via a reference outside the process
> context with mmget_not_zero() then go on to kthread_use_mm() via that
> reference.
>
> That invariant was broken by io_uring code (see previous sparc64 fix),
> but I'll point Fixes: to the original powerpc commit because we are
> changing that assumption going forward, so this will make backports
> match up.
>
> Fix this by no longer relying on that assumption, but by having each CPU
> check the mm is not being used, and clearing their own bit from the mask
> if it's okay. This fix relies on commit 38cf307c1f20 ("mm: fix
> kthread_use_mm() vs TLB invalidate") to disable irqs over the mm switch,
> and ARCH_WANT_IRQS_OFF_ACTIVATE_MM to be enabled.

You could use:

Depends-on: 38cf307c1f20 ("mm: fix kthread_use_mm() vs TLB invalidate")

> Fixes: 0cef77c7798a7 ("powerpc/64s/radix: flush remote CPUs out of single-threaded mm_cpumask")
> Signed-off-by: Nicholas Piggin <[email-masked]>
> ---
>  arch/powerpc/include/asm/tlb.h       | 13 -------------
>  arch/powerpc/mm/book3s64/radix_tlb.c | 23 ++++++++++++++++-------
>  2 files changed, 16 insertions(+), 20 deletions(-)

One minor nit below if you're respinning anyway.

You know this stuff better than me, but I still reviewed it and it seems
good to me.

Reviewed-by: Michael Ellerman <[email-masked]>

> diff --git a/arch/powerpc/include/asm/tlb.h b/arch/powerpc/include/asm/tlb.h
> index fbc6f3002f23..d97f061fecac 100644
> --- a/arch/powerpc/include/asm/tlb.h
> +++ b/arch/powerpc/include/asm/tlb.h
> @@ -66,19 +66,6 @@ static inline int mm_is_thread_local(struct mm_struct *mm)
>  		return false;
>  	return cpumask_test_cpu(smp_processor_id(), mm_cpumask(mm));
>  }
> -static inline void mm_reset_thread_local(struct mm_struct *mm)
> -{
> -	WARN_ON(atomic_read(&mm->context.copros) > 0);
> -	/*
> -	 * It's possible for mm_access to take a reference on mm_users to
> -	 * access the remote mm from another thread, but it's not allowed
> -	 * to set mm_cpumask, so mm_users may be > 1 here.
> -	 */
> -	WARN_ON(current->mm != mm);
> -	atomic_set(&mm->context.active_cpus, 1);
> -	cpumask_clear(mm_cpumask(mm));
> -	cpumask_set_cpu(smp_processor_id(), mm_cpumask(mm));
> -}
>  #else /* CONFIG_PPC_BOOK3S_64 */
>  static inline int mm_is_thread_local(struct mm_struct *mm)
>  {
> diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c b/arch/powerpc/mm/book3s64/radix_tlb.c
> index 0d233763441f..a421a0e3f930 100644
> --- a/arch/powerpc/mm/book3s64/radix_tlb.c
> +++ b/arch/powerpc/mm/book3s64/radix_tlb.c
> @@ -645,19 +645,29 @@ static void do_exit_flush_lazy_tlb(void *arg)
>  	struct mm_struct *mm = arg;
>  	unsigned long pid = mm->context.id;
>  
> +	/*
> +	 * A kthread could have done a mmget_not_zero() after the flushing CPU
> +	 * checked mm_users == 1, and be in the process of kthread_use_mm when
                                ^
                                in mm_is_singlethreaded()

Adding that reference would help join the dots for a new reader I think.

cheers

> +	 * interrupted here. In that case, current->mm will be set to mm,
> +	 * because kthread_use_mm() setting ->mm and switching to the mm is
> +	 * done with interrupts off.
> +	 */
>  	if (current->mm == mm)
> -		return; /* Local CPU */
> +		goto out_flush;
>  
>  	if (current->active_mm == mm) {
> -		/*
> -		 * Must be a kernel thread because sender is single-threaded.
> -		 */
> -		BUG_ON(current->mm);
> +		WARN_ON_ONCE(current->mm != NULL);
> +		/* Is a kernel thread and is using mm as the lazy tlb */
>  		mmgrab(&init_mm);
> -		switch_mm(mm, &init_mm, current);
>  		current->active_mm = &init_mm;
> +		switch_mm_irqs_off(mm, &init_mm, current);
>  		mmdrop(mm);
>  	}
> +
> +	atomic_dec(&mm->context.active_cpus);
> +	cpumask_clear_cpu(smp_processor_id(), mm_cpumask(mm));
> +
> +out_flush:
>  	_tlbiel_pid(pid, RIC_FLUSH_ALL);
>  }
>  
> @@ -672,7 +682,6 @@ static void exit_flush_lazy_tlbs(struct mm_struct *mm)
>  	 */
>  	smp_call_function_many(mm_cpumask(mm), do_exit_flush_lazy_tlb,
>  				(void *)mm, 1);
> -	mm_reset_thread_local(mm);
>  }
>  
>  void radix__flush_tlb_mm(struct mm_struct *mm)