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SubjectCONFIG_PREEMPT x86 assembly question
Date
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Hello

Whily lazy-examining kernel code, I found the following interesting point.

In arch/i386/kernel/entry.S

...
ENTRY(resume_kernel)
cmpl $0,TI_preempt_count(%ebp) # non-zero preempt_count ?
jnz restore_all
need_resched:
movl TI_flags(%ebp), %ecx # need_resched set ?
testb $_TIF_NEED_RESCHED, %cl
jz restore_all
testl $IF_MASK,EFLAGS(%esp) # interrupts off (exception path) ?
jz restore_all
movl $PREEMPT_ACTIVE,TI_preempt_count(%ebp)
sti
call schedule
movl $0,TI_preempt_count(%ebp)
cli
jmp need_resched
#endif
...

Why, after return from schedule(), first 0 is written to
TI_preempt_count(%ebp), and only then interrupts are disabled?
Wht not the reverse order?

As far as I understand, the idea of the preempt_count flag is to avoid
nested preemts. The fact that flag is reset before interrupts are
disabled, somewhat breaks this: interrupt may happen just after flag is
reset, causing nested interrupt while preempt_count flag is already reset.
In a very unprobable case this could happen unlimited number of times,
causing kernel stack overflow.

Very unprobable? But couldn't this be the cause of kernel lockups I
suffered several times while writing DVD on a probably broken media (which
could cause interrupt storm)?..
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