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blocking software interrupt thread & reentrancy

I have a question about software interrupt threads (__HAVE_FAST_SOFTINTS
case). I'm interrested in the mips implementation but I looked at x86
and didn't find what I looked for either, so my question applies to

When a software interrupt is triggered, the CPU switches to
the related kernel thread by changing the curlwp and stack pointers
(softint_fast_dispatch() for mips, Xsoftintr(),for x86).
This is triggered by a bit set in a bitmask by softint_trigger().
For both mips and x86, this bit is cleared from the bitmask
before invoking softint_fast_dispatch or Xsoftintr.

Now, if the softint thread blocks (this is allowed), another thread will
get scheduled and the IPL will go back to IPL_NONE. I can't see what prevents
another soft interrupt to be requested and the softint thread that
is blocking from being switched to again, overwriting the blocking context.
I'm probably missing something (I can't believe x86 avoided the issue
by luck for so long), can someone helps me ?

Manuel Bouyer <>
     NetBSD: 26 ans d'experience feront toujours la difference

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