Re: [Qemu-devel] [PATCH v2 3/5] qemu-queue: drop QCIRCLEQ

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/13/2012 05:44 PM, Peter Maydell wrote:
>> The main advantage of circular lists (the fact that the head node
>> >  has the same memory layout as any other node) is completely negated
>> >  by the implementation in qemu-queue.h.  Not surprisingly, nobody
>> >  uses QCIRCLEQ.  While this might change if RCU is ever adopted by
>> >  QEMU, the QLIST is also RCU-friendly and in fact it is used in a
>> >  RCU-like manner by 9pfs already.  So, just kill QCIRCLEQ.
> Kirk McKusick on why CIRCLEQ existed in the first place and
> why BSD still has it:
> http://markmail.org/message/i5oir4jhmkopjzy5
> ...basically just legacy back-compat. So it's fine for us to drop
> it from QEMU, since we have nothing to be back-compat with.

Thanks for posting this.

I think it's not entirely correct because the cast in QTAILQ_PREV and 
QTAILQ_FOREACH_REVERSE does not look like valid ANSI C.  No matter how 
hard I look I admit I cannot figure out how it works, but anyway I 
suspect it can be changed to ANSI C using typeof if one was bitten by 
it.  So removing QCIRCLEQ is not a bad idea anyway.

BTW, NetBSD also has some STAILQ which looks entirely the same as SIMPLEQ.

Paolo