Date:   Wed, 13 Feb 2019 14:47:44 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Tobin C. Harding" <tobin@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xarray: Document erasing entries during iteration

On Tue, Feb 12, 2019 at 05:51:29AM -0800, Matthew Wilcox wrote:
>On Tue, Feb 12, 2019 at 06:29:58PM +1100, Tobin C. Harding wrote:
>> I had my first go using the XArray today and during that I wondered if
>> it was safe to remove items during iteration.  Conceptually it seems
>> fine and it seemed to work just fine in code - is this something people
>> should not be doing for any reason?  Is this the best way to traverse
>> the tree and get every thing just to erase it?  Are we even supposed to
>> be thinking this is a tree or should we just be thinking it is an array?
>
>You should be thinking it's an array.  I've done everything I can to
>hide the fact that it's implemented as a tree because it's conceptually
>an array.
>
>The xa_for_each() iterator is designed to be extremely robust, at the
>expense of some performance.  The only state it keeps is the @index,
>so you can do anything you like to the XArray during the iteration.
>
>It's definitely worth being clearer in the documentation, for
>the benefit of people who're wondering what the equivalent of
>list_for_each_entry_safe() is.  So I'll apply this patch in a day or
>two unless anybody has further comment on it.
>
>> (As you might have guessed I _still_ don't know exactly how a radix tree
>> works :)
>
>That is _fine_.  As you know I hope to get rid of the radix tree soon ;-)
>

You mean replace radix tree in whole kernel? That would be a big effort.

BTW, have we compared the performance difference?

>> Oh, and FTR the XArray is hot - good effort man.
>> 
>> thanks,
>> Tobin.
>> 
>> 
>>  Documentation/core-api/xarray.rst | 3 ++-
>>  include/linux/xarray.h            | 2 ++
>>  2 files changed, 4 insertions(+), 1 deletion(-)
>> 
>> diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst
>> index 5d54b27c6eba..2578e0bdaa17 100644
>> --- a/Documentation/core-api/xarray.rst
>> +++ b/Documentation/core-api/xarray.rst
>> @@ -97,7 +97,8 @@ You can copy entries out of the XArray into a plain array by calling
>>  :c:func:`xa_extract`.  Or you can iterate over the present entries in
>>  the XArray by calling :c:func:`xa_for_each`.  You may prefer to use
>>  :c:func:`xa_find` or :c:func:`xa_find_after` to move to the next present
>> -entry in the XArray.
>> +entry in the XArray.  It is safe to call :c:func:`xa_release` on entries
>> +as you iterate over the array using :c:func:`xa_for_each`.
>
>that's spelled `xa_erase` ;-)
>
>>  Calling :c:func:`xa_store_range` stores the same entry in a range
>>  of indices.  If you do this, some of the other operations will behave
>> diff --git a/include/linux/xarray.h b/include/linux/xarray.h
>> index 5d9d318bcf7a..1f8974281a0a 100644
>> --- a/include/linux/xarray.h
>> +++ b/include/linux/xarray.h
>> @@ -407,6 +407,8 @@ static inline bool xa_marked(const struct xarray *xa, xa_mark_t mark)
>>   * you should use the xas_for_each() iterator instead.  The xas_for_each()
>>   * iterator will expand into more inline code than xa_for_each().
>>   *
>> + * It is safe to erase entries from the XArray as you iterate over it.
>> + *
>>   * Context: Any context.  Takes and releases the RCU lock.
>>   */
>>  #define xa_for_each(xa, index, entry) \
>> -- 
>> 2.20.1
>> 

-- 
Wei Yang
Help you, Help me

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