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Re: kern/42455: tstile hang with nfs

>  >  next=0x0 => mb is not a chained mbuf.
>  >  
>  >  nwc.mbuf_count is initialized with 1
>  >  and nwc.mbuf_count is decreased by one right before it enters
>  >  the while loop with 'cv_wait' so ctxt.nwc_mbuf_count must have
>  >  been 2 before it entered the loop.
>  >  
>  >  There is only one way to increase nwc.mbuf_count: it must run
>  >  the path where MEXTADD() is used which chains an mbuf to 'mb'.
>  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.

nfs_subs.c:968 is the line in error:

           do {
                    m2 = m_get(M_WAIT, MT_DATA);
                    MCLAIM(m2, m1->m_owner);
                    if (left >= MINCLSIZE) {
                            MCLGET(m2, M_WAIT);
                    m2->m_next = *nextp; <-- BUG happens here!!
                    *nextp = m2;
                    nextp = &m2->m_next;


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