fine grained locking and traversing linked lists
gurney_j at resnet.uoregon.edu
Fri Sep 3 16:27:20 PDT 2004
Maksim Yevmenkin wrote this message on Fri, Sep 03, 2004 at 15:04 -0700:
> >If you implement this, I strongly recommend making the lists singally
> >linked to avoid the possiablity of this deadlock.
> yes, i was thinking the same too. but double-linked list gives o(1)
> deletion (which is very nice :)
> so i guess the restrictions are:
> 1) lock order: item "x", then item "x->previous" (if any), then item
> "x->next" (if any)
> 2) always scan list forward
> 3) never use "x->previuos" field (except for "x" locking)
> it can even be done with standard sys/queue.h macros :) i just ran
> wintess test and it only complained about "acquiring duplicate lock of
> same type".
This can still lead to a dead lock:
obja <-> objb <-> objc <-> objd
T1: lock (objb), lock(obja)
T2: lock (objc), lock(objb, but blocks for T1)
T1: lock (objc, blocks for T2)
So, one solution is to use a list lock, and a refcnt'd object. Then
all of the next/previous pointers are protected by the list lock, and
the list gets one ref... Since you're using refcnts, you can keep a
ref, unlock the object lock, lock the list lock, before relocking the
object lock if necessary...
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
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