[GSoC2012]the problem of one-rbtree approach in POSIX Key project

Fri Jun 15 15:34:04 UTC 2012

On Fri, Jun 15, 2012 at 6:02 AM, Ashi <ashi08104 at gmail.com> wrote:
> Hi, all.
> Last two weeks, I'm working on the one-rbtree approach in the POSIX Key
> project. There are several reasons that I start to code without deciding
> which approach is best(there are several approaches have been discussed
> before, here[0] is the summary I have done before): first, I want to make
> myself more clear about the problem lies behind the POSIX Key by coding
> first. Second, we may also need to implement several different approaches
> and evaluate them finally. This post is a partly summary of one-rbtree
> approach and also with many problems in it. By the way, after try to
> implement the one-rbtree approach, I find the approach manage POSIX Key data
> only by Object manager(also described in [0]) is really immature. I've tried
> to implemented that before starting the one-rbtree approach, however, I'm
> stuck and turn to one-rbtree approach.
>
Can you expand on why the object manager approach does not work?

>
> There are 6 scenarios which we need deal with in POSIX key:
> 0. Key manage initialization
> 1. Key create
> 2. Key setspecific
> 3. Key getspecific
> 4. Key delete
> 5. Thread delete
>
> In the one-rbtree approach, I've done the 0~4(I've committed the code to
> one_rbtree branch in github repo[1]) , and the 5th thread delete hasn't been
> done, and needs more discussion(I'll list the problem below):
> 0. Key manage initialization
> I added two data structures in key.h, the POSIX_Keys_Rbtree_node is the node
> of the rbtree which holds all keys' values, and use Key and Thread_id member
> as rbtree key. POSIX_Keys_List_node is the node of a list which is used to
> save all nodes in one key or all nodes in one thread.
>
> typedef struct {
>   /** This field is the rbtree node structure. */
>   RBTree_Node Node;
Structure field names are usually lower-case, so "node".

>   /** This field is the POSIX key used as an rbtree key */
>   pthread_key_t Key;
>   /** This field is the Thread id also used as an rbtree key */
>   Objects_Id Thread_id;
>   /** This field points to the POSIX key value of specific thread */
>   void *Value;
>  }  POSIX_Keys_Rbtree_node;
>
> typedef struct POSIX_Keys_List_node_ {
>   /** This field is the pointer which points to the next node in the list */
>   struct POSIX_Keys_List_node_ *Next;
>   /** This field is the key of list node */
>   POSIX_Keys_Rbtree_node *Rbnode;
>  }  POSIX_Keys_List_node;
>
Instead of writing your own linked list you should use RTEMS Chains
(if you need a linked list).

> and revises the POSIX_Keys_Control structure, it has a member Head, which
> points to the list of all node in specific key. The reason I attach a list
> of nodes in one key is that when key deletion happens, I can delete all the
> key nodes from the rbtree in that key by traverse the list. I wonder is
> there any better way to get all nodes belong one key from the rbtree?
>
Does key delete remove all the keys having the same pthread_key_t, or
only those keys belonging to the executing thread? If only the one key
needs to be removed then I don't see the need for this list at all,
you would just "extract" the one key.

> typedef struct {
>    /** This field is the Object control structure. */
>    Objects_Control     Object;
>    /** This field is the data destructor. */
>    void (*destructor) (void *);
>    /** This field is the head of key's node list */
>    POSIX_Keys_List_node *Head;
>  }  POSIX_Keys_Control;
>
> The work during the POSIX Key manager initialization is:
> - besides the _Objects_Initialize_information, the global rbtree
> _POSIX_Keys_Rbtree is initialized to an empty rbtree.
>
> 1. Key create
> work in key create:
> - allocate the POSIX_Keys_Control object and initialize it.
>
> 2. Key setspecific
> work in key setspecific:
> - allocate a POSIX_Keys_Rbtree_node object and a POSIX_Keys_List_node object
> from RTEMS workspace
> - add the rbtree node to the rbtree
> - add the list node to the key's list
> problem:
> I'm not really clear about the workspace and heap memory, I just refer some
> code in other POSIX manager, all of them used _Workspace_Allocate and no
> malloc there. Could anyone explain the difference between them? Or is there
> any doc about that?
>
Workspace is for allocation of kernel objects. Not sure if there is an
explanation somewhere, although probably there ought to be...
Workspace is basically like heap (malloc), except that the workspace
is constrained in size based on application configuration. These
objects are rightly allocated from the workspace.

> 3. Key getspecific
> work in key getspecific:
> - get the node in the rbtree
> problem:
> - I find a _Thread_Enable_dispatch() function in the current keygetspecfic.c
> file, and don't know what is it used for.
>
_POSIX_Keys_Get implicitly obtains a dispatch disable lock (via
Objects_Get). This is releasing the lock. If your code does not use
Objects_Get then probably it does not need to enable dispatching...
although you are likely having to protect the global rbtree somehow
with a critical section?

> 4. Key delete
> work in key delete:
> - deallocate all nodes of specific key by iterate the key node list, and
> then delete it from rbtree, also delete it from the list
> - deallocate the POSIX_Key_Control object
>
Again unclear to me if the key delete should apply to all keys with
the same pthread_key_t or only the one for a particular thread. We
need to get an answer to this question before any decision can be made
about the best path forward.

> 5. Thread delete
> I find when a posix thread deleted, a _POSIX_Keys_Run_destructors() function
> runs, then all key's data in the deleted thread is destructed. I have some
> idea about it, but haven't implement it. I'm thinking add the necessary
> information about key to the thread's API_Extensions area. Actually, I add a
> list head pointer to the POSIX_API_Control structure, the pointer points to
> a list of nodes which contains all the key's value pointer in one thread.
> When thread exits, we can deallocate all the key data by traverse this list
> and then delete the whole list. And we need add node to this list when key
> setspecific. When Key deletes nothing should be done on this list. I think
> we needn't delete key nodes of deleted thread from rbtree when thread
> deleted, and the only work should be done when thread deleted is
> deallocating the key's data in that thread, right?
>
I think a chain pointing to all of a thread's keys could work fine...
although this thread-deletion requirement makes me think that every
thread could just have an rbtree of its own keys. The overhead of an
rbtree versus a chain is not a lot, and this approach would save
having a separate list for each thread's keys. You would just use a
thread's "local" keys rbtree. I'm not sure you would even need locking
in this case... interesting.

>
> thanks for any reply on this!
>
> links:
> [0]https://docs.google.com/document/d/1orDfUJdnxbxizClFNcOVbh0t3T0vtmt5FYJETSNb6KE/edit
> [1]https://github.com/ashi08104/rtems/tree/one_rbtree#
> --
> Best wishes!
> Zhongwei Yao
>
>
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