Re: Multiple potential races on vma->vm_flags

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Sep 09, 2015 at 08:58:26PM -0400, Sasha Levin wrote:
> On 09/07/2015 07:40 AM, Kirill A. Shutemov wrote:
> > On Sun, Sep 06, 2015 at 03:21:05PM -0400, Sasha Levin wrote:
> >> > ==================================================================
> >> > ThreadSanitizer: data-race in munlock_vma_pages_range
> >> > 
> >> > Write of size 8 by thread T378 (K2633, CPU3):
> >> >  [<ffffffff81212579>] munlock_vma_pages_range+0x59/0x3e0 mm/mlock.c:425
> >> >  [<ffffffff81212ac9>] mlock_fixup+0x1c9/0x280 mm/mlock.c:549
> >> >  [<ffffffff81212ccc>] do_mlock+0x14c/0x180 mm/mlock.c:589
> >> >  [<     inlined    >] SyS_munlock+0x74/0xb0 SYSC_munlock mm/mlock.c:651
> >> >  [<ffffffff812130b4>] SyS_munlock+0x74/0xb0 mm/mlock.c:643
> >> >  [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71
> >> > arch/x86/entry/entry_64.S:186
> > ...
> > 
> >> > Previous read of size 8 by thread T398 (K2623, CPU2):
> >> >  [<ffffffff8121d198>] try_to_unmap_one+0x78/0x4f0 mm/rmap.c:1208
> >> >  [<     inlined    >] rmap_walk+0x147/0x450 rmap_walk_file mm/rmap.c:1540
> >> >  [<ffffffff8121e7b7>] rmap_walk+0x147/0x450 mm/rmap.c:1559
> >> >  [<ffffffff8121ef72>] try_to_munlock+0xa2/0xc0 mm/rmap.c:1423
> >> >  [<ffffffff81211bb0>] __munlock_isolated_page+0x30/0x60 mm/mlock.c:129
> >> >  [<ffffffff81212066>] __munlock_pagevec+0x236/0x3f0 mm/mlock.c:331
> >> >  [<ffffffff812128a0>] munlock_vma_pages_range+0x380/0x3e0 mm/mlock.c:476
> >> >  [<ffffffff81212ac9>] mlock_fixup+0x1c9/0x280 mm/mlock.c:549
> >> >  [<ffffffff81212ccc>] do_mlock+0x14c/0x180 mm/mlock.c:589
> >> >  [<     inlined    >] SyS_munlock+0x74/0xb0 SYSC_munlock mm/mlock.c:651
> >> >  [<ffffffff812130b4>] SyS_munlock+0x74/0xb0 mm/mlock.c:643
> >> >  [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71
> >> > arch/x86/entry/entry_64.S:186
> > Okay, the detected race is mlock/munlock vs. rmap.
> > 
> > On rmap side we check vma->vm_flags in few places without taking
> > vma->vm_mm->mmap_sem. The vma cannot be freed since we hold i_mmap_rwsem
> > or anon_vma_lock, but nothing prevent vma->vm_flags from changing under
> > us.
> > 
> > In this particular case, speculative check in beginning of
> > try_to_unmap_one() is fine, since we re-check it under mmap_sem later in
> > the function.
> 
> So you're suggesting that this isn't the cause of the bad page flags
> error observed by Andrey and myself?

I don't see it, but who knows.

-- 
 Kirill A. Shutemov

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