possible ERESTARTNOHAND leak into userspace

[Michal Hocko <mhocko@suse.cz>]

Hi,
this has already been discussed few years back with reports that select
returned with ERESTARTNOHAND in multi-threaded applications
(http://forum.soft32.com/linux/PATCH-select-fix-sys_select-leak-ERESTARTNOHAND-userspace-ftopict338572.html)

Dave has come up with a possible explanation of the race but there was
no further follow up with a conclusion.

Just for reference:
        Thread_A				Thread_B
	CPU0					CPU1
						syscall_XYZ
core_sys_select				
  ret = -ERESTARTNOHAND;
  if (signal_pending(current))
     					do_notify_resume
     					  do_signal (clear signal pending)
  return ret;
return from syscall
no pending signal
return ERESTARTNOHAND

The race window is rather small and hard to trigger but we have seen
reports where people really saw select returning ERESTARTNOHAND (on
2.6.16 based kernel - x86_64).
I am not able to reproduce that myself neither with .16 kernel nor with
the current vanilla so I am not sure whether the problem has been fixed
already. But I do not see what prevents the race with vanilla.

Is the issue real and fixable?

Thanks
-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic