Re: [Qemu-devel] [PATCH v2] block: avoid SIGUSR2

[Kevin Wolf]

Am 28.10.2011 13:50, schrieb Paolo Bonzini:
> On 10/28/2011 01:33 PM, Kevin Wolf wrote:
>> I'm afraid that we can only avoid things like this reliably if we
>> convert all devices to be direct users of AIO/coroutines. The current
>> block layer infrastructure doesn't emulate the behaviour of bdrv_read
>> accurately as bottom halves can be run in the nested main loop.
>>
>> For floppy, the following seems to be a quick fix (Lucas, Cleber, does
>> this solve your problems?), though it's not very satisfying. And I'm not
>> quite sure yet why it doesn't always happen with kill() in
>> posix-aio-compat.c.
> 
> Another "fix" is to change idle bottom halves (at least the one in 
> hw/dma.c) to 10ms timers.

Which would be using the fact that timers are only executed in the real
main loop. Which makes me wonder if it would be enough for floppy if we
changed qemu_bh_poll() to take a bool run_idle_bhs that would be true in
the main loop and false an qemu_aio_wait().

Still this wouldn't be a general solution as normal BHs have the very
same problem if they are scheduled before a bdrv_read/write call. To
solve that I guess we'd have to reintroduce AsyncContext, but it has its
own problems and was removed for a reason.

Or we make some serious effort now to convert devices to AIO.

Kevin