From c7d65a78fc18ed70353baeb7497ec71a7c775ac5 Mon Sep 17 00:00:00 2001 From: Steven Rostedt Date: Thu, 7 Jun 2012 11:03:00 -0400 Subject: [PATCH] x86: Remove cmpxchg from i386 NMI nesting code I've been informed by someone on LWN called 'slashdot' that some i386 machines do not support a true cmpxchg. The cmpxchg used by the i386 NMI nesting code must be a true cmpxchg as disabling interrupts will not work for NMIs (which is the work around for i386s that do not have a true cmpxchg). This 'slashdot' character also suggested a fix to the issue. As the state of the nesting NMIs goes as follows: NOT_RUNNING -> EXECUTING EXECUTING -> NOT_RUNNING EXECUTING -> LATCHED LATCHED -> EXECUTING Having these states as enum values of: NOT_RUNNING = 0 EXECUTING = 1 LATCHED = 2 Instead of a cmpxchg to make EXECUTING -> NOT_RUNNING a dec_and_test() would work as well. If the dec_and_test brings the state to NOT_RUNNING, that is the same as a cmpxchg succeeding to change EXECUTING to NOT_RUNNING. If a nested NMI were to come in and change it to LATCHED, the dec_and_test() would convert the state to EXECUTING (what we want it to be in such a case anyway). I asked 'slashdot' to post this as a patch, but it never came to be. I decided to do the work instead. Thanks to H. Peter Anvin for suggesting to use this_cpu_dec_and_return() instead of local_dec_and_test(&__get_cpu_var()). Link: http://lwn.net/Articles/484932/ Cc: H. Peter Anvin Signed-off-by: Steven Rostedt --- Reading git-format-patch failed