Re: kern/42455: tstile hang with nfs

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

hi,

> 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

how is it a problem?  nwc_mbufcount is not incremented for the mbuf
allocated by nfsm_disct.

> => nfs_writerpc() calls cv_wait() which waits forever.

it waits for the sending mbuf chain being consumed.  it's a separate mbuf
chain from the one nfsm_disct works on.

if i were you, i'd look for mbuf leak in the underlying network stack
and driver.  sprinkling MCLAIM might help.

YAMAMOTO Takashi

> 
> Christoph