x86-32/irq: Don't switch to irq stack for a user-mode irq
authorLinus Torvalds <torvalds@linux-foundation.org>
Sun, 19 Feb 2012 19:46:36 +0000 (11:46 -0800)
committerIngo Molnar <mingo@elte.hu>
Mon, 20 Feb 2012 08:30:18 +0000 (09:30 +0100)
If the irq happens in user mode, our kernel stack is empty
(apart from the pt_regs themselves, of course), so there's no
need or advantage to switch.

And it really doesn't save any stack space, quite the reverse:
it means that a nested interrupt cannot switch irq stacks. So
instead of saving kernel stack space, it actually causes the
potential for *more* stack usage.

Also simplify the preemption count copy when we do switch
stacks: just copy the whole preemption count, rather than just
the softirq parts of it.  There is no advantage to the partial
copy: it is more effort to get a less correct result.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/alpine.LFD.2.02.1202191139260.10000@i5.linux-foundation.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

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