Re: kern/49305: deadlocks on sparc64 SMP

[Dennis Ferguson <dennis.c.ferguson%gmail.com@localhost>]

The following reply was made to PR kern/49305; it has been noted by GNATS.

From: Dennis Ferguson <dennis.c.ferguson%gmail.com@localhost>
To: gnats-bugs%NetBSD.org@localhost
Cc: kern-bug-people%netbsd.org@localhost,
 gnats-admin%netbsd.org@localhost,
 netbsd-bugs%netbsd.org@localhost,
 macallan%netbsd.org@localhost
Subject: Re: kern/49305: deadlocks on sparc64 SMP
Date: Thu, 30 Oct 2014 08:18:48 -0700

 On 29 Oct, 2014, at 20:25 , Masao Uebayashi <uebayasi%gmail.com@localhost> wrote:
 > If that's true, a missing membar_consumer() before:
 > 
 > http://nxr.netbsd.org/xref/src/sys/kern/subr_pcq.c#165
 > 
 > otherwise pcq_items[] values may be cached?
 
 I don't think so.  A membar_consumer() would ensure the
 read at line 165 was ordered after previous reads, with
 the read at line 159 being the only relevant one.  In this
 case, though, the read at 165 is dependent on the read
 at 159 (it uses the result from 159 to determine what to
 read at 165) and this is normally sufficient to ensure
 correct ordering without a barrier.  I'm pretty sure the
 only machine where a barrier would change anything here
 is a multiprocessor DEC Alpha, which had an odd cache
 arrangement; for everything else it is okay the way it
 is.