Re: [PATCH] kernel: fix data race in put_pid

From	Oleg Nesterov <oleg@redhat.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] kernel: fix data race in put_pid
Date	2015-09-17 20:20 +0200
Message-ID	<q9LWN-38s-1@gated-at.bofh.it> (permalink)
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Organization	linux.* mail to news gateway

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On 09/17, Dmitry Vyukov wrote:
>
> I can update the patch description, but let me explain it here first.

Yes thanks.

> Here is the essence of what happens:

Aha, so you really meant that 2 put_pid's can race with each other,

> // thread 1
> 1: pid->foo = 1; // foo is the first word of pid object
> // then it does put_pid
> 2: atomic_dec_and_test(&pid->count) // decrements count to 1 and
> returns false so the function returns
>
> // thread 2
> // executes put_pid
> 3: atomic_load(&pid->count); // returns 1, so proceed to kmem_cache_free
> // then kmem_cache_free does:
> 5: head->freelist = (void*)pid;
>
> This can be executed as:
>
> 4: *(void**)pid = head->freelist;
> 1: pid->foo = 1; // foo is the first word of pid object
> 2: atomic_dec_and_test(&pid->count) // decrements count to 1 and
> returns false so the function returns
> 3: atomic_load(&pid->count); // returns 1, so proceed to kmem_cache_free
> 5: head->freelist = (void*)pid;

Unless I am totally confused, everything is simpler. We can forget
about the hoisting, freelist, etc.

Thread 2 can see the result of atomic_dec_and_test(), but not the
result of "pid->foo = 1". In this case in can free the object which
can be re-allocated _before_ STORE(pid->foo) completes. Of course,
this would be really bad.

I need to recheck, but afaics this is not possible. This optimization
is fine, but probably needs a comment. We rely on delayed_put_pid()
called by RCU. And note that nobody can write to this pid after it
is removed from the rcu-protected list.

So I think this is false alarm, but I'll try to recheck tomorrow, it
is too late for me today.

Oleg.

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Thread

[PATCH] kernel: fix data race in put_pid Dmitry Vyukov <dvyukov@google.com> - 2015-09-17 15:30 +0200
  Re: [PATCH] kernel: fix data race in put_pid Oleg Nesterov <oleg@redhat.com> - 2015-09-17 18:20 +0200
    Re: [PATCH] kernel: fix data race in put_pid Dmitry Vyukov <dvyukov@google.com> - 2015-09-17 18:50 +0200
      Re: [PATCH] kernel: fix data race in put_pid Oleg Nesterov <oleg@redhat.com> - 2015-09-17 19:50 +0200
        Re: [PATCH] kernel: fix data race in put_pid Dmitry Vyukov <dvyukov@google.com> - 2015-09-17 20:00 +0200
          Re: [PATCH] kernel: fix data race in put_pid Dmitry Vyukov <dvyukov@google.com> - 2015-09-17 20:10 +0200
          Re: [PATCH] kernel: fix data race in put_pid Oleg Nesterov <oleg@redhat.com> - 2015-09-17 20:20 +0200
            Re: [PATCH] kernel: fix data race in put_pid Dmitry Vyukov <dvyukov@google.com> - 2015-09-17 20:40 +0200
            Re: [PATCH] kernel: fix data race in put_pid Peter Zijlstra <peterz@infradead.org> - 2015-09-18 11:00 +0200
              Re: [PATCH] kernel: fix data race in put_pid Peter Zijlstra <peterz@infradead.org> - 2015-09-18 11:10 +0200
                Re: [PATCH] kernel: fix data race in put_pid Peter Zijlstra <peterz@infradead.org> - 2015-09-18 11:40 +0200
                Re: [PATCH] kernel: fix data race in put_pid James Hogan <james.hogan@imgtec.com> - 2015-09-18 14:40 +0200
                Re: [PATCH] kernel: fix data race in put_pid Peter Zijlstra <peterz@infradead.org> - 2015-09-18 14:50 +0200
                Re: [PATCH] kernel: fix data race in put_pid "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-09-18 18:40 +0200
              Re: [PATCH] kernel: fix data race in put_pid Dmitry Vyukov <dvyukov@google.com> - 2015-09-18 11:10 +0200
                Re: [PATCH] kernel: fix data race in put_pid Will Deacon <will.deacon@arm.com> - 2015-09-18 11:30 +0200
                Re: [PATCH] kernel: fix data race in put_pid Peter Zijlstra <peterz@infradead.org> - 2015-09-18 11:40 +0200
                Re: [PATCH] kernel: fix data race in put_pid Peter Zijlstra <peterz@infradead.org> - 2015-09-18 13:30 +0200
                Re: [PATCH] kernel: fix data race in put_pid Will Deacon <will.deacon@arm.com> - 2015-09-18 13:40 +0200
                Re: [PATCH] kernel: fix data race in put_pid Dmitry Vyukov <dvyukov@google.com> - 2015-09-18 14:00 +0200
                Re: [PATCH] kernel: fix data race in put_pid Peter Zijlstra <peterz@infradead.org> - 2015-09-18 14:10 +0200
                Re: [PATCH] kernel: fix data race in put_pid Peter Zijlstra <peterz@infradead.org> - 2015-09-18 14:30 +0200
                Re: [PATCH] kernel: fix data race in put_pid Will Deacon <will.deacon@arm.com> - 2015-09-18 14:50 +0200
                Re: [PATCH] kernel: fix data race in put_pid Peter Zijlstra <peterz@infradead.org> - 2015-09-18 15:20 +0200
                Re: [PATCH] kernel: fix data race in put_pid Oleg Nesterov <oleg@redhat.com> - 2015-09-18 15:50 +0200
                Re: [PATCH] kernel: fix data race in put_pid Peter Zijlstra <peterz@infradead.org> - 2015-09-18 16:00 +0200
                Re: [PATCH] kernel: fix data race in put_pid Dmitry Vyukov <dvyukov@google.com> - 2015-09-18 16:00 +0200
                Re: [PATCH] kernel: fix data race in put_pid Oleg Nesterov <oleg@redhat.com> - 2015-09-18 16:50 +0200
                Re: [PATCH] kernel: fix data race in put_pid Dmitry Vyukov <dvyukov@google.com> - 2015-09-22 10:40 +0200
                Re: [PATCH] kernel: fix data race in put_pid Eric Dumazet <eric.dumazet@gmail.com> - 2015-09-18 18:20 +0200
                Re: [PATCH] kernel: fix data race in put_pid Peter Zijlstra <peterz@infradead.org> - 2015-09-18 18:30 +0200
                Re: [PATCH] kernel: fix data race in put_pid "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-09-18 18:00 +0200
              Re: [PATCH] kernel: fix data race in put_pid Oleg Nesterov <oleg@redhat.com> - 2015-09-18 15:40 +0200
              Re: [PATCH] kernel: fix data race in put_pid Oleg Nesterov <oleg@redhat.com> - 2015-09-18 15:40 +0200
                Re: [PATCH] kernel: fix data race in put_pid Peter Zijlstra <peterz@infradead.org> - 2015-09-18 16:00 +0200
                Re: [PATCH] kernel: fix data race in put_pid Oleg Nesterov <oleg@redhat.com> - 2015-09-18 17:10 +0200
                Re: [PATCH] kernel: fix data race in put_pid Oleg Nesterov <oleg@redhat.com> - 2015-09-18 17:40 +0200
                Re: [PATCH] kernel: fix data race in put_pid "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-09-18 18:30 +0200
      Re: [PATCH] kernel: fix data race in put_pid "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-09-17 20:00 +0200

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