Re: [PATCH v5] mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared

[Cyrill Gorcunov]

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?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/