[PATCH] jbd: do_get_write_access lock contention reduction
We're seeing heavy contention against j_list_lock on 8-way in do_get_write_access(). We actually don't need j_list_lock in there except for one little case - the per-bh jbd_lock_bh_state() is sufficient to protect this buffer's internal state. On some nice quick LVM array Ram Pai measured an overall 3x speedup from this patch: the script took the following time on 265mm1 real 0m57.504s user 0m0.400s sys 7m29.867s and with the 2patches it took real 0m19.983s user 0m0.438s sys 1m55.896s
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