Question about interrupt enabling/disabling in kernel exit path

From: Lee Schermerhorn <>
Date: 2005-11-10 05:12:05

I have a question about enabling interrupts in the kernel exit path.
Especially, in do_notify_resume_user().  I have work that I need to
defer to this point, but it needs to run with interrupts enabled.  I
wanted to piggy back off TIF_NOTIFY_RESUME, because that is more or less
generic code and gets handled in a C function where I can play the usual
header games:  static inline wrapper function when feature is
configured;  null macro when not.

I see a comment at the beginning of ia64_leave_kernel that says that
"work.need_resched, etc. mustn't get changed by this CPU before it
returns to user- or fsys-mode, hence we disable interrupts early on."
[This may be a stale comment.  I.e., we now check for resched needed in
current task's thread_info flags.] But, then I see later that when
CONFIG_PREEMPT is set, we do enable interrupts and then re-disable them.

Then I notice that after we return from do_notify_resume_user() we don't
recheck TIF_NEED_RESCHED, etc. in the thread_info flags.   So, maybe
this is the issue?  If I open an interrupt window in
do_notify_resume_user(), 'NEED_RESCHED might get set and we wouldn't
notice it on return to the assembly language code.  [I might even sleep
here.  Is that a problem?]

I took a look at the x86_64 and i386 versions and they always goes back
to check for resched after calling their version of do_notify_resume()
[no '_user suffix on the routine].  If this were the only issue with
enabling interrupts in do_notify_resume_user(), I could change it to
return non-zero if recheck is needed [zero in the usual case] and then
go back and recheck thread_info flags.


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Received on Thu Nov 10 05:12:56 2005

This archive was generated by hypermail 2.1.8 : 2005-11-10 05:13:05 EST