Re: kern/42455: tstile hang with nfs

[Christoph Egger <Christoph_Egger%gmx.de@localhost>]

The following reply was made to PR kern/42455; it has been noted by GNATS.

From: Christoph Egger <Christoph_Egger%gmx.de@localhost>
To: gnats-bugs%NetBSD.org@localhost
Cc: YAMAMOTO Takashi <yamt%mwd.biglobe.ne.jp@localhost>, 
 kern-bug-people%netbsd.org@localhost, gnats-admin%netbsd.org@localhost, 
 netbsd-bugs%netbsd.org@localhost
Subject: Re: kern/42455: tstile hang with nfs
Date: Thu, 28 Oct 2010 07:06:41 +0200

 On 28.10.10 06:50, YAMAMOTO Takashi wrote:
 > The following reply was made to PR kern/42455; it has been noted by GNATS.
 > 
 > From: yamt%mwd.biglobe.ne.jp@localhost (YAMAMOTO Takashi)
 > To: Christoph_Egger%gmx.de@localhost
 > Cc: gnats-bugs%NetBSD.org@localhost, netbsd-bugs%netbsd.org@localhost, 
 > gnats-admin%netbsd.org@localhost,
 >      kern-bug-people%netbsd.org@localhost
 > Subject: Re: kern/42455: tstile hang with nfs
 > Date: Thu, 28 Oct 2010 04:48:41 +0000 (UTC)
 > 
 >  hi,
 >  
 >  >>  hi,
 >  >>  
 >  >>  >>  >  I added some more debug lines and figured out that the macro
 >  >>  >>  >  nfsm_wcc_data() drops the mbuf chain w/o decreasing
 >  >>  >>  >  ctxt.nwc_mbufcount.
 >  >>  >>  
 >  >>  >>  The nfsm_wcc_data() macro calls the nfsm_postop_attr() macro.
 >  >>  >>  The nfsm_postop_attr() macro calls nfsm_loadattrcache() function.
 >  >>  >>  The nfsm_loadattrcache() function calls nfsm_disct() function.
 >  >>  >>  
 >  >>  >>  nfsm_disct() is the function in error which drops the mbuf chain.
 >  >>  
 >  >>  are you sure?
 >  > 
 >  > yes, absolutely and reproducable.
 >  > 
 >  >>  iirc, nwc_mbufcount is about sending mbuf.  otoh, nfsm_disct
 >  >>  is for received mbuf.
 >  > 
 >  > nfs_writerpc *does* call nfsm_disct() through nfsm_wcc_data,
 >  > nfsm_postop_attr and nfsm_loadattrcache in this order.
 >  > 
 >  > So you are saying this should never happen?
 >  
 >  i'm saying i don't understand.
 >  
 >  nfs_writerpc sends a request to the server, using mreq and mb.
 >  it's what nwc_mbufcount is used for.
 >  
 >  it then parses the reply from the server, using mrep and md.
 >  it's what nfsm_wcc_data/nfsm_postop_attr/nfsm_loadattrcache/nfsm_disct are
 >  used for.
 
 Ah, I see.
 
 >  i don't understand how a problem in the latter causes the nwc_mbufcount
 >  problem.  the above two are somehow mixed up?
 
 nfsm_disct() creates new mbufs with m_get() and MCLAIM().
 nfs_writerpc() relies on that the ext hook is called on m_free.
 
 But nfsm_disct() does *not* use MEXTADD(), so the ext hook is empty.
 => nfs_writerpc_extfree() won't be called to decrement nwc_mbufcount
 => nfs_writerpc() calls cv_wait() which waits forever.
 
 Christoph