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CVS commit: [netbsd-7-0] src/sys/kern

Module Name:    src
Committed By:   martin
Date:           Tue May 15 04:48:16 UTC 2018

Modified Files:
        src/sys/kern [netbsd-7-0]: uipc_mbuf.c

Log Message:
Pull up following revision(s) (requested by maxv in ticket #1602):

        sys/kern/uipc_mbuf.c: revision 1.211 (via patch)

Modify m_defrag, so that it never frees the first mbuf of the chain. While
here use the given 'flags' argument, and not M_DONTWAIT.

We have a problem with several drivers: they poll an mbuf chain from their
queues and call m_defrag on them, but m_defrag could update the mbuf
pointer, so the mbuf in the queue is no longer valid. It is not easy to
fix each driver, because doing pop+push will reorder the queue, and we
don't really want that to happen.

This problem was independently spotted by me, Kengo, Masanobu, and other
people too it seems (perhaps PR/53218).

Now m_defrag leaves the first mbuf in place, and compresses the chain
only starting from the second mbuf in the chain.

It is important not to compress the first mbuf with hacks, because the
storage of this first mbuf may be shared with other mbufs.

To generate a diff of this commit:
cvs rdiff -u -r1. -r1. src/sys/kern/uipc_mbuf.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.

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