Re: [Qemu-devel] linux-user NPTL problem with signals

[Gary Thomas <gary@mlbassoc.com>]

On 09/23/2009 03:51 PM, Gary Thomas wrote:
> I'm running a multi-threaded program (linux-user on i386).  I
> have a problem with signals; when one arrives, the running
> thread abruptly changes :-(
>
> Anyone have any ideas about this?
>

I've tracked this down to the fact that there should be a
unique value for the GS segment register (at least) per thread.
This doesn't seem to be the case and when a signal is dispatched,
the GS segment register is updated with [probably] the most recent
value created by a new thread, not the running thread.

Hence, I see this:
   qemu: got signal 17, pthread: 0x62289840
   queue_signal: sig=17, pthread: 0x62289840
   qemu: process signal 17
   setup_sigcontext.757 (pthread: 0x62289840) - GS: 0x33/0x43e6f110
   do_sigreturn, pthread: 0x62289840
   restore_sigcontext.973 (pthread: 0x62289840) - GS: 0x33/0x43e6f110
   cpu_x86_load_seg_cache.727 - pthread: 0x62289840, GS: 0x33/0x454c3b70
   restore_sigcontext.975 (pthread: 0x62289840) - GS: 0x33/0x454c3b70

The GS values printed are the 'selector' and 'base' respectively.
The 'base' value happens to be the Linux pthread_self() [for the
emulated process].  You can see that this is switching threads
but the system has not really switched threads...

Any help?

-- 
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Gary Thomas                 |  Consulting for the
MLB Associates              |    Embedded world
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