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9390ef0c85
percpu_rw_semaphore->writer_mutex was only added to simplify the initial rewrite, the only thing it protects is clear_fast_ctr() which otherwise could be called by multiple writers. ->rw_sem is enough to serialize the writers. Kill this mutex and add "atomic_t write_ctr" instead. The writers increment/decrement this counter, the readers check it is zero instead of mutex_is_locked(). Move atomic_add(clear_fast_ctr(), slow_read_ctr) under down_write() to avoid the race with other writers. This is a bit sub-optimal, only the first writer needs this and we do not need to exclude the readers at this stage. But this is simple, we do not want another internal lock until we add more features. And this speeds up the write-contended case. Before this patch the racing writers sleep in synchronize_sched_expedited() sequentially, with this patch multiple synchronize_sched_expedited's can "overlap" with each other. Note: we can do more optimizations, this is only the first step. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Cc: Anton Arapov <anton@redhat.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Michal Marek <mmarek@suse.cz> Cc: Mikulas Patocka <mpatocka@redhat.com> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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