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There are 64 requests in progress: > map->word = 0xFFFFFFFFFFFFFFFF > 2. After all the 64 requests complete, and no more requests come: > map->word = 0xFFFFFFFFFFFFFFFF, map->cleared = 0xFFFFFFFFFFFFFFFF > 3. Now two tasks try to allocate requests: > T1: T2: > __blk_mq_get_tag . > __sbitmap_queue_get . > sbitmap_get . > sbitmap_find_bit . > sbitmap_find_bit_in_word . > __sbitmap_get_word -> nr=-1 __blk_mq_get_tag > sbitmap_deferred_clear __sbitmap_queue_get > /* map->cleared=0xFFFFFFFFFFFFFFFF */ sbitmap_find_bit > if (!READ_ONCE(map->cleared)) sbitmap_find_bit_in_word > return false; __sbitmap_get_word -> nr=-1 > mask = xchg(&map->cleared, 0) sbitmap_deferred_clear > atomic_long_andnot() /* map->cleared=0 */ > if (!(map->cleared)) > return false; > /* > * map->cleared is cleared by T1 > * T2 fail to acquire the tag > */ > > 4. T2 is the sole tag waiter. When T1 puts the tag, T2 cannot be woken > up due to the wake_batch being set at 6. If no more requests come, T1 > will wait here indefinitely. > > This patch achieves two purposes: > 1. Check on ->cleared and update on both ->cleared and ->word need to > be done atomically, and using spinlock could be the simplest solution. > So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which > may cause potential race. > > 2. Add extra check in sbitmap_deferred_clear(), to identify whether > ->word has free bits. > > Fixes: 661d4f55a794 ("sbitmap: remove swap_lock") Is it blamed right? Considering that the revert alone doesn't fix the problem, it sounds like the 2nd step might need to be ported to kernels even without the blamed commit. -- Pavel Begunkov