linux-kernel - Re: [PATCH 3/3] userfaultfd: selftest: recycle lock threads first

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Date:   Sat, 29 Sep 2018 13:32:52 +0300
From:   Mike Rapoport <rppt@...ux.vnet.ibm.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
        Jerome Glisse <jglisse@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        Zi Yan <zi.yan@...rutgers.edu>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        linux-kselftest@...r.kernel.org, Shaohua Li <shli@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] userfaultfd: selftest: recycle lock threads first

On Sat, Sep 29, 2018 at 04:43:11PM +0800, Peter Xu wrote:
> Now we recycle the uffd servicing threads earlier than the lock
> threads.  It might happen that when the lock thread is still blocked at
> a pthread mutex lock while the servicing thread has already quitted for
> the cpu so the lock thread will be blocked forever and hang the test
> program.  To fix the possible race, recycle the lock threads first.
> 
> This never happens with current missing-only tests, but when I start to
> run the write-protection tests (the feature is not yet posted upstream)
> it happens every time of the run possibly because in that new test we'll
> need to service two page faults for each lock operation.
> 
> Signed-off-by: Peter Xu <peterx@...hat.com>

Acked-by: Mike Rapoport <rppt@...ux.vnt.ibm.com>

> ---
>  tools/testing/selftests/vm/userfaultfd.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
> index f79706f13ce7..a388675b15af 100644
> --- a/tools/testing/selftests/vm/userfaultfd.c
> +++ b/tools/testing/selftests/vm/userfaultfd.c
> @@ -623,6 +623,12 @@ static int stress(unsigned long *userfaults)
>  	if (uffd_test_ops->release_pages(area_src))
>  		return 1;
> 
> +
> +	finished = 1;
> +	for (cpu = 0; cpu < nr_cpus; cpu++)
> +		if (pthread_join(locking_threads[cpu], NULL))
> +			return 1;
> +
>  	for (cpu = 0; cpu < nr_cpus; cpu++) {
>  		char c;
>  		if (bounces & BOUNCE_POLL) {
> @@ -640,11 +646,6 @@ static int stress(unsigned long *userfaults)
>  		}
>  	}
> 
> -	finished = 1;
> -	for (cpu = 0; cpu < nr_cpus; cpu++)
> -		if (pthread_join(locking_threads[cpu], NULL))
> -			return 1;
> -
>  	return 0;
>  }
> 
> -- 
> 2.17.1
> 

-- 
Sincerely yours,
Mike.