svn commit: r363854 - head/sys/kern

[Mateusz Guzik]

Author: mjg
Date: Tue Aug  4 19:52:16 2020
New Revision: 363854
URL: https://svnweb.freebsd.org/changeset/base/363854

Log:
  rms: add a comment explaining performance deficiencies of write locking

Modified:
  head/sys/kern/kern_rmlock.c

Modified: head/sys/kern/kern_rmlock.c
==============================================================================
--- head/sys/kern/kern_rmlock.c	Tue Aug  4 19:44:43 2020	(r363853)
+++ head/sys/kern/kern_rmlock.c	Tue Aug  4 19:52:16 2020	(r363854)
@@ -872,6 +872,10 @@ db_show_rm(const struct lock_object *lock)
  * Neither rms_rlock nor rms_runlock use fences. Instead compiler barriers are
  * inserted to prevert reordering of generated code. Execution ordering is
  * provided with the use of an IPI handler.
+ *
+ * No attempt is made to track which CPUs read locked at least once,
+ * consequently write locking sends IPIs to all of them. This will become a
+ * problem at some point. The easiest way to lessen it is to provide a bitmamp.
  */
 
 void