Commits
Peter Zijlstra committed d4d74af4b87
RFC: FROMLIST: locking/percpu-rwsem: Optimize readers and reduce global impact Currently the percpu-rwsem switches to (global) atomic ops while a writer is waiting; which could be quite a while and slows down releasing the readers. This patch cures this problem by ordering the reader-state vs reader-count (see the comments in __percpu_down_read() and percpu_down_write()). This changes a global atomic op into a full memory barrier, which doesn't have the global cacheline contention. This also enables using the percpu-rwsem with rcu_sync disabled in order to bias the implementation differently, reducing the writer latency by adding some cost to readers. Mailing-list-URL: https://lkml.org/lkml/2016/8/9/181 Cc: Paul McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> [jstultz: Backported to 4.4] Change-Id: I8ea04b4dca2ec36f1c2469eccafde1423490572f Signed-off-by: John Stultz <john.stultz@linaro.org>