Re: kern/42455: tstile hang with nfs

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

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