On Sun, 30 Apr 2017, Taylor R Campbell wrote:
Date: Sun, 30 Apr 2017 13:36:12 +0000 (UTC) From: christos%astron.com@localhost (Christos Zoulas) How frequently is this going to be used and what is the performance penalty for it. I am asking because I am running LOCKDEBUG all the time.The idea is that it will be used unconditionally before potentially low-probability conditional branches that acquire the lock. So it may be appropriate to limit its use to KDASSERT, since it means no-lock fast paths don't get to be no-lock fast paths.
The only place it is currently planned to be used is in the new localcount(9) code. For each call to device_lookup_acquire(), there would be one corresponding call to device_release(), and that's where this would be used. So, for frequency of use, once for each time a driver looks up the device_t.
As for penalty, it should be very low. The expectation is that there would be no contention for the lock, so no waiting for mutex_enter() to finish. Just the cost of taking and releasing the mutex.
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