From: Nicholas Piggin <npiggin@gmail.com>
To: linux-mm@kvack.org, Michael Ellerman <mpe@ellerman.id.au>
Cc: Andrew Morton <akpm@linux-foundation.org>,
"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
Jens Axboe <axboe@kernel.dk>,
"David S. Miller" <davem@davemloft.net>,
linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
linuxppc-dev@lists.ozlabs.org,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 4/4] powerpc/64s/radix: Fix mm_cpumask trimming race vs kthread_use_mm
Date: Wed, 02 Sep 2020 19:48:48 +1000 [thread overview]
Message-ID: <1599040088.z7acx6fvvf.astroid@bobo.none> (raw)
In-Reply-To: <87pn751zcb.fsf@mpe.ellerman.id.au>
Excerpts from Michael Ellerman's message of September 1, 2020 10:00 pm:
> Nicholas Piggin <npiggin@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")
Good idea I wil.
>> Fixes: 0cef77c7798a7 ("powerpc/64s/radix: flush remote CPUs out of single-threaded mm_cpumask")
>> Signed-off-by: Nicholas Piggin <npiggin@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 <mpe@ellerman.id.au>
Thanks.
>
>> 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.
Yes you're right I can change that.
Thanks,
Nick
prev parent reply other threads:[~2020-09-02 9:48 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-28 10:00 [PATCH 0/4] more mm switching vs TLB shootdown and lazy tlb Nicholas Piggin
2020-08-28 10:00 ` [PATCH 1/4] mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race Nicholas Piggin
2020-08-28 11:15 ` peterz
2020-08-31 1:25 ` Nicholas Piggin
2020-08-28 10:00 ` [PATCH 2/4] powerpc: select ARCH_WANT_IRQS_OFF_ACTIVATE_MM Nicholas Piggin
2020-08-28 10:00 ` [PATCH 3/4] sparc64: remove mm_cpumask clearing to fix kthread_use_mm race Nicholas Piggin
2020-08-28 10:00 ` [PATCH 4/4] powerpc/64s/radix: Fix mm_cpumask trimming race vs kthread_use_mm Nicholas Piggin
2020-09-01 12:00 ` Michael Ellerman
2020-09-02 9:48 ` Nicholas Piggin [this message]
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