Nicholas Piggin <npig...@gmail.com> 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 <npig...@gmail.com>
> ---
> 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 <m...@ellerman.id.au>
> 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)
