Re: [LTP] [PATCH] doc: document tst_checkpoint_create and tst_checkpoint_init

[Cyril Hrubis <chrubis@suse.cz>]

Hi!
> I thought we had this issue only in parent:
> 
> ---------------------------------------------------------------------------------------------------
>                 parent                        |          child
>                                               | tst_checkpoint_child_wait
>                                               |    - has to wait until parent opens pipe for write
> tst_checkpoint_signal_child (NONBLOCK write)  | 
> tst_checkpoint_parent_wait (NONBLOCK read)    |
>   - parent now races on read with child       |
> ---------------------------------------------------------------------------------------------------
> 
> because child is opening always in blocking mode, it will always wait until other
> side is opened as well:
> ---------------------------------------------------------------------------------------------------
>                 parent                        |          child
>                                               | tst_checkpoint_signal_parent
>                                               |    - has to wait until parent opens pipe for read
> tst_checkpoint_parent_wait (NONBLOCK read)    |
>   - reads from FIFO                           |
>                                               | tst_checkpoint_child_wait
>                                               |    - has to wait until parent opens pipe for write
> tst_checkpoint_signal_child (NONBLOCK write)  |   
>   - at this point first byte was already read |
>     from FIFO, parent writes second byte      |
>                                               |    - and only now will child succeed opening pipe
> ---------------------------------------------------------------------------------------------------
> 
> Am I missing something?

You are right. The problem here is that the fifo can be opened for
NONBLOCK reading even if there are no writers and therefore there are
two readers that fight for the data in the fifo.

The second ordering should be fine as far as I can tell.

-- 
Cyril Hrubis
chrubis@suse.cz

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