Hi,

I’ve run into trouble with Vladimir’s async backup series on MacOS, 
namely that iotest 256 fails with qemu exiting because of a SIGUSR2.

Turns out this is because MacOS (-xcode) uses coroutine-sigaltstack, 
when I use this on Linux, I get the same error.

(You can find the series applied on my block branch e.g. here:

https://github.com/XanClic/qemu.git block
)

Some debugging later I found that the problem seems to be two threads 
simultaneously creating a coroutine.  It makes sense that this case 
would appear with Vladimir’s series and iotest 256, because 256 runs two 
backup jobs in two different threads in a transaction, i.e. they’re 
launched simultaneously.  The async backup series makes backup use many 
concurrent coroutines and so by default launches 64+x coroutines when 
the backup is started.  Thus, the case of two coroutines created 
concurrently in two threads is very likely to occur.

I think the problem is in coroutine-sigaltstack’s qemu_coroutine_new(). 
It sets up a SIGUSR2 handler, then changes the signal handling stack, 
then raises SIGUSR2, then reverts the signal handling stack and the 
SIGUSR2 handler.  As far as I’m aware, setting up signal handlers and 
changing the signal handling stack are both process-global operations, 
and so if two threads do so concurrently, they will interfere with each 
other.  What usually happens is that one thread sets up everything, 
while the other is already in the process of reverting its changes: So 
the second thread reverts the SIGUSR2 handler to the default, and then 
the first thread raises SIGUSR2, thus making qemu exit.

(Could be worse though.  Both threads could set up the sigaltstack, then 
both raise SIGUSR2, and then we get one coroutine_trampoline() 
invocation in each thread, but both would use the same stack.  But I 
don’t think I’ve ever seen that happen, presumably because the race time 
window is much shorter.)

Now, this all seems obvious to me, but I’m wondering...  If 
coroutine-sigaltstack really couldn’t create coroutines concurrently, 
why wouldn’t we have noticed before?  I mean, this new backup case is 
kind of a stress test, yes, but surely we would have seen the problem 
already, right?  That’s why I’m not sure whether my analysis is correct.

Anyway, I’ve attached a patch that wraps the whole SIGUSR2 handling 
section in a mutex, and that makes 256 pass reliably with Vladimir’s 
async backup series.  Besides being unsure whether the problem is really 
in coroutine-sigaltstack, I also don’t know whether getting out the big 
guns and wrapping everything in the mutex is the best solution.  So, 
it’s an RFC, I guess.

Max