[PATCH 23/46] mm/userfaultfd: write-lock the VMA before waking waiters when unregistering

[sashiko-bot at kernel.org]

> userfaultfd_unregister() wakes pending missing faults before clearing the
> VMA's userfaultfd registration. With per-VMA fault locking, holding
> mmap_lock for write does not prevent another fault from queuing after
> the wakeup but before userfaultfd_clear_vma() write-locks the VMA. That
> waiter can then remain asleep indefinitely.
> 
> Call vma_start_write() before waking waiters. Existing fault handlers
> queue themselves before dropping the VMA read lock, and new readers are
> excluded until mmap_write_unlock().
> 
> Fixes: 29a22b9e08d7 ("mm: handle userfaults under VMA lock")
> Cc: stable at vger.kernel.org
> Reported-by: Josef Bacik <josef at toxicpanda.com>
> Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>

Sashiko has reviewed this patch and found no issues.

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