Re: [Xen-devel] [PATCH 6/7] libxl: fork: Provide ..._always_selective_reap

[Jim Fehlig <jfehlig@xxxxxxxx>]

Ian Jackson wrote:
> Jim Fehlig writes ("Re: [PATCH 6/7] libxl: fork: Provide 
> ..._always_selective_reap"):
>   
>> Ian Jackson wrote:
>>     
>>> +    /* libxl owns SIGCHLD all the time, but it must only reap its own
>>> +     * children.  The application will reap its own children
>>> +     * synchronously with waitpid, without the assistance of SIGCHLD. */
>>> +    libxl_sigchld_owner_libxl_always_selective_reap,
>>>       
>> Should there be some documentation in the opening comments of
>> "Subprocess handling"?  E.g. an entry under "For programs which run
>> their own children alongside libxl's:"?
>>     
>
> Yes.
>
>   
>> BTW, it is not clear to me how to use libxl_childproc_setmode() wrt
>> different libxl_ctx.  Currently in the libvirt libxl driver there's a
>> driver-wide ctx for more host-centric operations like
>> libxl_get_version_info(), libxl_get_free_memory(), etc., and a
>> per-domain ctx for domain-specific operations.  The current doc for
>> libxl_childproc_setmode() says:
>>     
>
> Oh dear.  Can you change it to use the same ctx everywhere ?
>   

What would be the downside of this in a multi-threaded application,
where there are many concurrent domain operations?  I was under the
impression that such an app would need a per-domain ctx.

Regards,
Jim

> If not, I'm afraid, someone needs to
>  - keep a record of all the libxl ctxs
>  - when the self-pipe triggers, iterate over all the ctxs calling
>    libxl_childproc_sigchld_occurred
>
> If that "someone" is libvirt, you'll have to do the self-pipe trick.
>
> It would probably be easier to have libxl maintain a global list of
> libxl ctx's for this purpose.
>
>   
>> When calling setmode() on the driver-wide or on each domain-specific
>> ctx, I get an assert with this hunk
>>
>> libvirtd: libxl_fork.c:241: libxl__sigchld_installhandler: Assertion
>> `!sigchld_owner' failed.
>>     
>
> The problem is that the SIGCHLD ownership is attached to the
> individual ctx.  This could in principle be changed, I think.
>
> I will try to see if I can do that.
>
> Ian.
>
>   

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