Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 14 Jul 2015 16:33:39 -0700 | Subject | Re: [tip:x86/asm] x86/irq, context_tracking: Document how IRQ context tracking works and add an RCU assertion |
| |
On Tue, Jul 14, 2015 at 4:26 PM, Frederic Weisbecker <fweisbec@gmail.com> wrote: > On Tue, Jul 07, 2015 at 03:54:32AM -0700, tip-bot for Andy Lutomirski wrote: >> Commit-ID: 0333a209cbf600e980fc55c24878a56f25f48b65 >> Gitweb: http://git.kernel.org/tip/0333a209cbf600e980fc55c24878a56f25f48b65 >> Author: Andy Lutomirski <luto@kernel.org> >> AuthorDate: Fri, 3 Jul 2015 12:44:34 -0700 >> Committer: Ingo Molnar <mingo@kernel.org> >> CommitDate: Tue, 7 Jul 2015 10:59:10 +0200 >> >> x86/irq, context_tracking: Document how IRQ context tracking works and add an RCU assertion >> >> Signed-off-by: Andy Lutomirski <luto@kernel.org> >> Cc: Andy Lutomirski <luto@amacapital.net> >> Cc: Borislav Petkov <bp@alien8.de> >> Cc: Brian Gerst <brgerst@gmail.com> >> Cc: Denys Vlasenko <dvlasenk@redhat.com> >> Cc: Denys Vlasenko <vda.linux@googlemail.com> >> Cc: Frederic Weisbecker <fweisbec@gmail.com> >> Cc: H. Peter Anvin <hpa@zytor.com> >> Cc: Kees Cook <keescook@chromium.org> >> Cc: Linus Torvalds <torvalds@linux-foundation.org> >> Cc: Oleg Nesterov <oleg@redhat.com> >> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> >> Cc: Peter Zijlstra <peterz@infradead.org> >> Cc: Rik van Riel <riel@redhat.com> >> Cc: Thomas Gleixner <tglx@linutronix.de> >> Cc: paulmck@linux.vnet.ibm.com >> Link: http://lkml.kernel.org/r/e8bdc4ed0193fb2fd130f3d6b7b8023e2ec1ab62.1435952415.git.luto@kernel.org >> Signed-off-by: Ingo Molnar <mingo@kernel.org> >> --- >> arch/x86/kernel/irq.c | 15 +++++++++++++++ >> 1 file changed, 15 insertions(+) >> >> diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c >> index 88b36648..6233de0 100644 >> --- a/arch/x86/kernel/irq.c >> +++ b/arch/x86/kernel/irq.c >> @@ -216,8 +216,23 @@ __visible unsigned int __irq_entry do_IRQ(struct pt_regs *regs) >> unsigned vector = ~regs->orig_ax; >> unsigned irq; >> >> + /* >> + * NB: Unlike exception entries, IRQ entries do not reliably >> + * handle context tracking in the low-level entry code. This is >> + * because syscall entries execute briefly with IRQs on before >> + * updating context tracking state, so we can take an IRQ from >> + * kernel mode with CONTEXT_USER. The low-level entry code only >> + * updates the context if we came from user mode, so we won't >> + * switch to CONTEXT_KERNEL. We'll fix that once the syscall >> + * code is cleaned up enough that we can cleanly defer enabling >> + * IRQs. >> + */ >> + > > Now is it a problem to take interrupts in kernel mode with CONTEXT_USER? > I'm not sure it's worth trying to make it not happen.
It's not currently a problem, but it would be nice if we could do the equivalent of:
if (user_mode(regs)) { user_exit(); (or enter_from_user_mode or whatever) } else { // don't bother -- already in CONTEXT_KERNEL }
i.e. the same thing that do_general_protection, etc do in -tip. That would get rid of any need to store the previous context.
Currently we can't because of syscalls and maybe because of KVM. KVM has a weird fake interrupt thing.
> >> entering_irq(); >> >> + /* entering_irq() tells RCU that we're not quiescent. Check it. */ >> + rcu_lockdep_assert(rcu_is_watching(), "IRQ failed to wake up RCU"); > > Why do we need to check that?
Sanity check. If we're changing a bunch of context tracking details, I want to assert that it actually works.
--Andy
| |