linux-kernel - Re: [PATCH -next] fork: annotate a data race in vm_area

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Message-Id: <EAD6E54D-8A57-4494-94F2-2EEEC3265560@lca.pw>
Date:   Mon, 17 Feb 2020 22:59:47 -0500
From:   Qian Cai <cai@....pw>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, elver@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] fork: annotate a data race in vm_area_dup()

> On Feb 17, 2020, at 5:31 PM, Kirill A. Shutemov <kirill@...temov.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?