blk-mq: Fix a race between bt_clear_tag() and bt_get()
What we need is the following two guarantees: * Any thread that observes the effect of the test_and_set_bit() by __bt_get_word() also observes the preceding addition of 'current' to the appropriate wait list. This is guaranteed by the semantics of the spin_unlock() operation performed by prepare_and_wait(). Hence the conversion of test_and_set_bit_lock() into test_and_set_bit(). * The wait lists are examined by bt_clear() after the tag bit has been cleared. clear_bit_unlock() guarantees that any thread that observes that the bit has been cleared also observes the store operations preceding clear_bit_unlock(). However, clear_bit_unlock() does not prevent that the wait lists are examined before that the tag bit is cleared. Hence the addition of a memory barrier between clear_bit() and the wait list examination. Signed-off-by: Bart Van Assche <bvanassche@acm.org> Cc: Christoph Hellwig <hch@lst.de> Cc: Robert Elliott <elliott@hp.com> Cc: Ming Lei <ming.lei@canonical.com> Cc: Alexander Gordeev <agordeev@redhat.com> Cc: <stable@vger.kernel.org> # v3.13+ Signed-off-by: Jens Axboe <axboe@fb.com>
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@ -158,7 +158,7 @@ restart:
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return -1;
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return -1;
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}
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}
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last_tag = tag + 1;
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last_tag = tag + 1;
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} while (test_and_set_bit_lock(tag, &bm->word));
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} while (test_and_set_bit(tag, &bm->word));
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return tag;
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return tag;
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}
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}
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@ -357,11 +357,10 @@ static void bt_clear_tag(struct blk_mq_bitmap_tags *bt, unsigned int tag)
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struct bt_wait_state *bs;
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struct bt_wait_state *bs;
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int wait_cnt;
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int wait_cnt;
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/*
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clear_bit(TAG_TO_BIT(bt, tag), &bt->map[index].word);
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* The unlock memory barrier need to order access to req in free
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* path and clearing tag bit
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/* Ensure that the wait list checks occur after clear_bit(). */
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*/
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smp_mb();
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clear_bit_unlock(TAG_TO_BIT(bt, tag), &bt->map[index].word);
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bs = bt_wake_ptr(bt);
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bs = bt_wake_ptr(bt);
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if (!bs)
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if (!bs)
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