Re: pserialize(9) vs. TAILQ

[Dennis Ferguson <dennis.c.ferguson%gmail.com@localhost>]

On 18 Nov, 2014, at 22:13 , Masao Uebayashi <uebayasi%gmail.com@localhost> wrote:
> In the TAILQ case, where readers iterate a list by TAILQ_FOREACH(),
> TAILQ_REMOVE() is safely used as the update operation, because:
> 
> - Readers only see tqe_next in TAILQ_FOREACH(), and
> - Pointer assignment (done in TAILQ_REMOVE()) is atomic.
> 
> If this is correct, pserialize(9) should be updated to be clearer;
> probably this TAILQ example is worth being added there.

I don't think TAILQ is a good example for this.  While TAILQ_REMOVE()
will work, none of the TAILQ_INSERT_*() macros can be reliably used
with concurrent readers so it isn't clear how the list you are
removing stuff from could have been built.

Also, the TAILQ macros depend on a type pun fairly similar to the one
that did in the CIRCLEQ macros and hence have a lifetime which extends
only to some future version of the compiler smart enough to recognize
that and miscompile them.  I think new code, and new man page text,
should probably avoid using the macros to keep from making the eventual
problem with them bigger than it already is.

Dennis Ferguson