Messages in this thread | | | Date | Sat, 7 Nov 2015 12:18:24 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH 1/4] x86/entry/64: Fix irqflag tracing wrt context tracking |
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On Fri, Nov 06, 2015 at 03:12:43PM -0800, Andy Lutomirski wrote: > Paolo pointed out that enter_from_user_mode could be called while > irqflags were traced as though IRQs were on. > > In principle, this could confuse lockdep. It doesn't cause any > problems that I've seen in any configuration, but if I build with > CONFIG_DEBUG_LOCKDEP=y, enable a nohz_full CPU, and add code like: > > if (irqs_disabled()) { > spin_lock(&something); > spin_unlock(&something); > } > > to the top of enter_from_user_mode, then lockdep will complain > without this fix. It seems that lockdep's irqflags sanity checks > are too weak to detect this bug without forcing the issue. > > This patch adds one byte to normal kernels, and it's IMO a bit ugly. > I haven't spotted a better way to do this yet, though. The issue is > that we can't do TRACE_IRQS_OFF until after SWAPGS (if needed), but > we're also supposed to do it before calling C code.
I would not mind to have that explanation in the code itself so that people don't scratch heads why the duplicated TRACE_IRQS_OFF call.
-- Regards/Gruss, Boris.
ECO tip #101: Trim your mails when you reply.
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