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Date: Tue, 26 Aug 2014 18:19:14 +0400
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Hugh Dickins <hughd@google.com>, Peter Feiner <pfeiner@google.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Pavel Emelyanov <xemul@parallels.com>, Jamie Liu <jamieliu@google.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Magnus Damm <magnus.damm@gmail.com>
Subject: Re: [PATCH v5] mm: softdirty: enable write notifications on VMAs
 after VM_SOFTDIRTY cleared
Message-ID: <20140826141914.GA8952@moon>
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On Tue, Aug 26, 2014 at 05:04:19PM +0300, Kirill A. Shutemov wrote:
> > > 
> > > But now I'm realizing that if this is the _only_ place which modifies
> > > vm_flags with down_read, then it's "probably" safe.  I've a vague
> > > feeling that this was discussed before - is that so, Cyrill?
> > 
> > Well, as far as I remember we were not talking before about vm_flags
> > and read-lock in this function, maybe it was on some unrelated lkml thread
> > without me CC'ed? Until I miss something obvious using read-lock here
> > for vm_flags modification should be safe, since the only thing which is
> > important (in context of vma-softdirty) is the vma's presence. Hugh,
> > mind to refresh my memory, how long ago the discussion took place?
> 
> It seems safe in vma-softdirty context. But if somebody else will decide that
> it's fine to modify vm_flags without down_write (in their context), we
> will get trouble. Sasha will come with weird bug report one day ;)
> 
> At least vm_flags must be updated atomically to avoid race in middle of
> load-modify-store.

Which race you mean here? Two concurrent clear-refs?