Re: kern/28448 (stackable filesystems locking breakage when looking up DOTDOT)

[yamt%mwd.biglobe.ne.jp@localhost (YAMAMOTO Takashi)]

The following reply was made to PR kern/28448; it has been noted by GNATS.

From: yamt%mwd.biglobe.ne.jp@localhost (YAMAMOTO Takashi)
To: gnats-bugs%NetBSD.org@localhost
Cc: kern-bug-people%netbsd.org@localhost, netbsd-bugs%netbsd.org@localhost, 
gnats-admin%netbsd.org@localhost,
        pooka%NetBSD.org@localhost
Subject: Re: kern/28448 (stackable filesystems locking breakage when looking
 up DOTDOT)
Date: Sat,  3 Jan 2009 13:11:59 +0900 (JST)

 hi,
 
 > Synopsis: stackable filesystems locking breakage when looking up DOTDOT
 > 
 > State-Changed-From-To: open->feedback
 > State-Changed-By: pooka%narn.netbsd.org@localhost
 > State-Changed-When: Sat, 19 Jan 2008 18:35:38 +0200
 > State-Changed-Why:
 > I don't understand the problem.  There are two cases:
 > 
 > 1) the lower layer exports a lock and there is no upper lock
 > 2) the lower layer does not export a lock pointer, in which case the
 >    relock dance in the lower layer will not use the upper layer's
 >    locks
 > 
 > Either way I don't see a deadlock between the lower and upper layer.
 
 this PR was about 2).
 it can deadlock between the parent and child directories
 in the upper layer due to the lack of dotdot relock dances in
 the upper layer.  the relock dance in the lower layer doesn't
 make sense because it's done keeping the upper layer vnode locked.
 
 YAMAMOTO Takashi