Should java/nio/channels/spi/AbstractInterruptibleChannel.java#isOpen be synchronized ?

[Sean Chou]

Forget to say, the bug appears on linux, and the fix is for linux. The
testcase doesn't reproduce on windows.


On Tue, Dec 11, 2012 at 8:13 PM, Sean Chou <zhouyx at linux.vnet.ibm.com>wrote:

> Hi,
>
> This is the detail problem, there is a small time window in which a 3
> threads race makes select() always return 0 without blocking.
>
> I wrote a testcase(http://cr.openjdk.java.net/~zhouyx/OJDK-714/webrev0.2/)
> but it needs to modify the lib to reproduce, because the time windows is
> small.
>
> The reproduce scenario is described in follow, use Tx for thread x:
>
> 1. T1 (the user code) is selecting a channel(suppose C), it just returns
> from native select function, and niolib select method is checking if the
> returned channel is interested in the event, then 2 happens;
> 2. T2 is closing channel C, it just set the open variable to false but not
> yet closed the channel actually, and then 3 happens;
> 3. T3 set the interedOps of the channel to 0. // 0 means the channel is
> not interested in anything, the channel will be put into cancel list
> normally.
>
> In this senario, T1 returns from select, and return 0 which means no
> channel is selected(because the channel C returned from native invocation
> has nothing insterested in, it is not returned to application). Then T1
> goes to invoke select again(usually in a loop, this is how select is
> designed to be used). In normal case, select method checks if any channels
> those should be cancelled and remove them from the set to be selected.
> Then, goes to native select function.
>
> The problem is: select method first checks if the channel is closed, if it
> is closed, select method doesn't put it into cancel list.
>
> In above senario, channel C is in close state, but not closed indeed, and
> setInteredOps to 0(which means cancel). So select method doesn't put C into
> cancel list(due to the problem) which means the native select set still
> contains channel C . So the native select always return C and nio select
> always return 0. Until the channel is finally closed.
>
>
> The testcase: http://cr.openjdk.java.net/~zhouyx/OJDK-714/webrev0.2/
>
> A working fix: http://cr.openjdk.java.net/~zhouyx/OJDK-714/webrev_fix/
>
>
> Please have a look.
>
>
> On Wed, Dec 5, 2012 at 6:10 PM, Alan Bateman <Alan.Bateman at oracle.com>wrote:
>
>>  On 05/12/2012 02:47, Sean Chou wrote:
>>
>> A small problem I'm still checking. So the closeLock is just to make sure
>> the channel is closed only once, is that right?
>>
>> This is how close is specified:
>>
>> "If this channel is already closed then invoking this method has no
>> effect.
>>
>> This method may be invoked at any time. If some other thread has already
>> invoked it, however, then another invocation will block until the first
>> invocation is complete, after which it will return without effect."
>>
>> -Alan.
>>
>>
>
>
> --
> Best Regards,
> Sean Chou
>
>


-- 
Best Regards,
Sean Chou
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