On Wed, Sep 09, 2015 at 05:27:16PM +0200, Vlastimil Babka wrote: > On 09/07/2015 01:40 PM, 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. > > > >False-negative is fine too here, since we will mlock the page in > >__mm_populate() on mlock side after mlock_fixup(). > > > >BUT. > > > >We *must* have all speculative vm_flags accesses wrapped READ_ONCE() to > >avoid all compiler trickery, like duplication vm_flags access with > >inconsistent results. > > Doesn't taking a semaphore, as in try_to_unmap_one(), already imply a > compiler barrier forcing vm_flags to be re-read? Yes, but it doesn't prevent compiler from generation multiple reads from vma->vm_flags and it may blow up if two values doesn't match. > >I looked only on VM_LOCKED checks, but there are few other flags checked > >in rmap. All of them must be handled carefully. At least READ_ONCE() is > >required. > > > >Other solution would be to introduce per-vma spinlock to protect > >vma->vm_flags and probably other vma fields and offload this duty > >from mmap_sem. > >But that's much bigger project. > > Sounds like an overkill, unless we find something more serious than this. May be... -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>