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From: Qian Cai <cai@lca.pw>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	elver@google.com, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next] fork: annotate a data race in vm_area_dup()
Date: Mon, 17 Feb 2020 22:59:47 -0500	[thread overview]
Message-ID: <EAD6E54D-8A57-4494-94F2-2EEEC3265560@lca.pw> (raw)
In-Reply-To: <20200217223138.doaph66iwprbwhw5@box>



> On Feb 17, 2020, at 5:31 PM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> 
> I'm confused. AFAICS both sides hold mmap_sem on write:
> 
> - vm_mmap_pgoff() takes mmap_sem for the write on the write side
> 
> - do_mprotect_pkey() takes mmap_sem for the write on the read side
> 
> 
> What do I miss?

Ah, good catch. I missed the locking for the read there. This is interesting because Marco
did confirmed that the concurrency could happen,

https://lore.kernel.org/lkml/20191025173511.181416-1-elver@google.com/

If that means KCSAN is not at fault, then I could think of two things,

1) someone downgrades the lock.

I don’t think that a case here. Only __do_munmap() will do that but I did not see how
it will affect us here.

2) the reader and writer are two different processes.

So, they held a different mmap_sem, but I can’t see how could two processes shared
the same vm_area_struct. Also, file->f_mapping->i_mmap was also stored in the
writer, but I can’t see how it was also loaded in the reader.

Any ideas?

  reply	other threads:[~2020-02-18  3:59 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-14 20:33 Qian Cai
2020-02-17 22:31 ` Kirill A. Shutemov
2020-02-18  3:59   ` Qian Cai [this message]
2020-02-18 10:30     ` Kirill A. Shutemov
2020-02-18 12:40       ` Qian Cai
2020-02-18 14:09         ` Marco Elver
2020-02-18 15:00           ` Qian Cai
2020-02-18 15:18             ` Kirill A. Shutemov
2020-02-18 16:46               ` Qian Cai

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