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cap on memory usage, then shared LOOKUP operations could start free'ing dirhash structures. Without these fixes, concurrent free's on the same directory could result in one of the threads blocked on a lock in a dirhash structure free'd by the other thread. - Replace the lockmgr lock in the dirhash structure with an sx lock. - Use a reference count managed with ufsdirhash_hold()/drop() to determine when to free the dirhash structures. The directory i-node holds a reference while the dirhash is attached to an i-node. Code that wishes to lock the dirhash while holding a shared vnode lock must first acquire a private reference to the dirhash while holding the vnode interlock before acquiring the dirhash sx lock. After acquiring the sx lock, it drops the private reference after checking to see if the dirhash is still used by the directory i-node.
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