PR/51632 CVS commit: src/sys/kern

["Ryota Ozaki" <ozaki-r%netbsd.org@localhost>]

The following reply was made to PR kern/51632; it has been noted by GNATS.

From: "Ryota Ozaki" <ozaki-r%netbsd.org@localhost>
To: gnats-bugs%gnats.NetBSD.org@localhost
Cc: 
Subject: PR/51632 CVS commit: src/sys/kern
Date: Mon, 21 Nov 2016 00:54:21 +0000

 Module Name:	src
 Committed By:	ozaki-r
 Date:		Mon Nov 21 00:54:21 UTC 2016
 
 Modified Files:
 	src/sys/kern: subr_xcall.c
 
 Log Message:
 Fix a race condition of low priority xcall
 
 xc_lowpri and xc_thread are racy and xc_wait may return during/before
 executing all xcall callbacks, resulting in a kernel panic at worst.
 
 xc_lowpri serializes multiple jobs by a mutex and a cv. If all xcall
 callbacks are done, xc_wait returns and also xc_lowpri accepts a next job.
 
 The problem is that a counter that counts the number of finished xcall
 callbacks is incremented *before* actually executing a xcall callback
 (see xc_tailp++ in xc_thread). So xc_lowpri accepts a next job before
 all xcall callbacks complete and a next job begins to run its xcall callbacks.
 
 Even worse the counter is global and shared between jobs, so if a xcall
 callback of the next job completes, the shared counter is incremented,
 which confuses wc_wait of the previous job as all xcall callbacks of the
 previous job are done and wc_wait of the previous job returns during/before
 executing its xcall callbacks.
 
 How to fix: there are actually two counters that count the number of finished
 xcall callbacks for low priority xcall for historical reasons (I guess):
 xc_tailp and xc_low_pri.xc_donep. xc_low_pri.xc_donep is incremented correctly
 while xc_tailp is incremented wrongly, i.e., before executing a xcall callback.
 We can fix the issue by dropping xc_tailp and using only xc_low_pri.xc_donep.
 
 PR kern/51632
 
 
 To generate a diff of this commit:
 cvs rdiff -u -r1.18 -r1.19 src/sys/kern/subr_xcall.c
 
 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.