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 19:50 +0200
Message-ID	<q9LtN-2jP-29@gated-at.bofh.it> (permalink)
References	<q9Hqa-4OV-5@gated-at.bofh.it> <q9K4G-np-11@gated-at.bofh.it> <q9KxJ-W9-47@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On 09/17, Dmitry Vyukov wrote:
>
> What happens here exactly matches what is described in CONTROL
> DEPENDENCIES section of Documentation/memory-barriers.txt. So all the
> bad things described there are possible here.

And I still can't understand how these bad things connect to put_pid().
Probably I should re-read memory-barriers.txt, it changes quite often.

> I don't
> know what to add to that.

OK, let me quote the parts of your changelog,

	For example, if store to the first word of the object to build a freelist
	in kmem_cache_free() hoists above the check, stores to the first word
	in other threads can corrupt the memory allocator freelist.

I simply can't parse this. Yes, this is probably because of my bad
English, but I'll appreciate it if you can explain at least, say,
"stores to the first word in other threads".

Did you mean that a freed pid can be reallocated by another thread,
then overwritten, and this all can happen before atomic_read(count)?

Hmm. or perhaps you meant that the "last" put_pid() which observes
atomic_read() == 1 can race with another thread which writes to this
pid and does put_pid()? This is another story, and if you meant this
the changelog could clearly explain your concerns.

Or what?

So let me repeat. Since I can't understand you, I leave this to other
reviewers. But imho the changelog should be updated in any case.

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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