----- Original Message -----
> From: "Cyril Hrubis" <[email protected]>
> To: "Jan Stancek" <[email protected]>
> Cc: [email protected]
> Sent: Thursday, 16 October, 2014 3:01:28 PM
> Subject: Re: [LTP] [PATCH] doc: document tst_checkpoint_create and
> tst_checkpoint_init
>
> We should also add:
>
> IMPORTANT: Be wary that using single checkpoint to signal child/parent
> followed
> immediately by wait for parent/child creates an race condtion
> between opening the pipe and reading from it (the process
> writing to the fifo may may be the wery same process that
> reads from it and the checkpoint code will exit the test
> with error). You are advised to use two checkpoints in this
> case.
>
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?
Regards,
Jan
> --
> Cyril Hrubis
> [email protected]
>
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