Re: [Qemu-devel] Re: [PATCH] Fix freezing bug in curses console

[Anthony Liguori <anthony@codemonkey.ws>]

Paul Brook wrote:
>>>> I think it still suffers from the same race condition so today it
>>>> wouldn't work.  You could fix the bottom half scheduling though so that
>>>> you could safely schedule a bottom half from a signal handler (using
>>>> roughly the same trick).
>>>>         
>>> Fwiw, it's perfectly sensible to have a single pipe which is shared by
>>> all signal handlers, just used to say "check for work flags set".
>>>       
>> And if you need the main loop to be able to distinguish signals coming
>> out of the pipe, then just write the signum into the pipe as a byte,
>> instead of a single dummy byte. Or even write the whole 'siginfo_t'
>> struct passed to the signal handler, and read it out in sizeof(siginfo_t)
>> sized chunks for processing.
>>     
>
> I don't think this will works. If the pipe buffer gets full the write will 
> either block or you'll loose signals.
>
> When using the pipe as a simple semaphore all you care about is the presence 
> or absence of data. It doesn't matter if subsequent writes loose data (e.g. 
> by not retrying a nonblocking write) as long as a write to an empty pipe 
> succeeds.
>   

Yup.  You need to use a global flag to distinguish the type of signal.

Regards,

Anthony Liguori

> Paul
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