On Tue, 4 Apr 2006, Ingo Molnar wrote:
>
> * Simon Derr <[email protected]> wrote:
>
> > On Tue, 28 Mar 2006, Ingo Molnar wrote:
> >
> > First issue: 'BUG: udev:45 task might have lost a preemption check!'
> >
> > When looking at the code in preempt_enable_no_resched(), why is the
> > value of preempt_count() checked to be non-zero _after_ calling
> > dec_preempt_count() ?
> >
> > I saw several posts on this list claiming that this message is
> > harmless, but I'd like to figure what's going on.
>
> the warning means that doing a preempt_enable_no_resched() while being
> in a preemptible section is most likely a bug, and that you could lose a
> need_resched() check. (and introduce a scheduling latency) What's the
> backtrace? This could be the sign of a not fully/correctly converted
> arch/*/kernel/process.c (but i'm only guessing here).
Wow, thanks for the fast reply !
The backtrace is:
[<a00000010007fc50>] preempt_enable_no_resched+0xb0/0xe0
[<a000000100037510>] disabled_fph_fault+0x110/0x140
[<a0000001000378e0>] ia64_fault+0x240/0x11c0
[<a00000010000be40>] ia64_leave_kernel+0x0/0x290
But I must be severely misunderstanding something.
What I understood is that in preemptible sections preempt_count() is
zero, and in non preemptible sections it is >0.
If preempt_count() is 1, then preempt_enable_no_resched() will decrement
it and issue a warning. This is what happens in disabled_fph_fault().
Where am I wrong ?
> you should first check the PREEMPT_NONE kernel with PREEMPT_SOFTIRQS and
> PREEMPT_HARDIRQS enabled. I.e. first check whether IRQ threading works.
> Then enable all the other PREEMPT options one by one: PREEMPT_DESKTOP,
> PREEMPT_RCU, PREEMPT_BKL. Only when all these work switch to PREEMPT_RT.
>
Thanks, I'll try that.
Simon.
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