Re: [PATCH v4 02/27] hardirq/nmi: Allow nested nmi_enter()

Peter Zijlstra
2020-02-24T16:13:51Z

On Fri, Feb 21, 2020 at 11:21:30PM +0100, Frederic Weisbecker wrote:
> On Fri, Feb 21, 2020 at 02:34:18PM +0100, Peter Zijlstra wrote:
> > Since there are already a number of sites (ARM64, PowerPC) that
> > effectively nest nmi_enter(), lets make the primitive support this
> > before adding even more.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Reviewed-by: Petr Mladek <pmladek@suse.com>
> > Acked-by: Will Deacon <will@kernel.org>
> > Acked-by: Marc Zyngier <maz@kernel.org>
> > ---
> > arch/arm64/include/asm/hardirq.h | 4 ++--
> > arch/arm64/kernel/sdei.c | 14 ++------------
> > arch/arm64/kernel/traps.c | 8 ++------
> > arch/powerpc/kernel/traps.c | 22 ++++++----------------
> > include/linux/hardirq.h | 5 ++++-
> > include/linux/preempt.h | 4 ++--
> > kernel/printk/printk_safe.c | 6 ++++--
> > 7 files changed, 22 insertions(+), 41 deletions(-)
> >
> > --- a/kernel/printk/printk_safe.c
> > +++ b/kernel/printk/printk_safe.c
> > @@ -296,12 +296,14 @@ static __printf(1, 0) int vprintk_nmi(co
> >
> > void notrace printk_nmi_enter(void)
> > {
> > - this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
> > + if (!in_nmi())
> > + this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
> > }
> >
> > void notrace printk_nmi_exit(void)
> > {
> > - this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
> > + if (!in_nmi())
> > + this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
> > }
>
> If the outermost NMI is interrupted while between printk_nmi_enter()
> and preempt_count_add(), there is still a risk that we race and clear?

Damn, true. That also means I need to fix the arm64 bits, and that's a
little more tricky.

Something like so perhaps.. hmm?

---
--- a/arch/arm64/include/asm/hardirq.h
+++ b/arch/arm64/include/asm/hardirq.h
@@ -32,30 +32,52 @@ u64 smp_irq_stat_cpu(unsigned int cpu);

struct nmi_ctx {
u64 hcr;
+ unsigned int cnt;
};

DECLARE_PER_CPU(struct nmi_ctx, nmi_contexts);

-#define arch_nmi_enter() \
- do { \
- if (is_kernel_in_hyp_mode() && !in_nmi()) { \
- struct nmi_ctx *nmi_ctx = this_cpu_ptr(&nmi_contexts); \
- nmi_ctx->hcr = read_sysreg(hcr_el2); \
- if (!(nmi_ctx->hcr & HCR_TGE)) { \
- write_sysreg(nmi_ctx->hcr | HCR_TGE, hcr_el2); \
- isb(); \
- } \
- } \
- } while (0)
+#define arch_nmi_enter() \
+do { \
+ struct nmi_ctx *___ctx; \
+ unsigned int ___cnt; \
+ \
+ if (!is_kernel_in_hyp_mode() || in_nmi()) \
+ break; \
+ \
+ ___ctx = this_cpu_ptr(&nmi_contexts); \
+ ___cnt = ___ctx->cnt; \
+ if (!(___cnt & 1) && __cnt) { \
+ ___ctx->cnt += 2; \
+ break; \
+ } \
+ \
+ ___ctx->cnt |= 1; \
+ barrier(); \
+ nmi_ctx->hcr = read_sysreg(hcr_el2); \
+ if (!(nmi_ctx->hcr & HCR_TGE)) { \
+ write_sysreg(nmi_ctx->hcr | HCR_TGE, hcr_el2); \
+ isb(); \
+ } \
+ barrier(); \
+ if (!(___cnt & 1)) \
+ ___ctx->cnt++; \
+} while (0)

-#define arch_nmi_exit() \
- do { \
- if (is_kernel_in_hyp_mode() && !in_nmi()) { \
- struct nmi_ctx *nmi_ctx = this_cpu_ptr(&nmi_contexts); \
- if (!(nmi_ctx->hcr & HCR_TGE)) \
- write_sysreg(nmi_ctx->hcr, hcr_el2); \
- } \
- } while (0)
+#define arch_nmi_exit() \
+do { \
+ struct nmi_ctx *___ctx; \
+ \
+ if (!is_kernel_in_hyp_mode() || in_nmi()) \
+ break; \
+ \
+ ___ctx = this_cpu_ptr(&nmi_contexts); \
+ if ((___ctx->cnt & 1) || (___ctx->cnt -= 2)) \
+ break; \
+ \
+ if (!(nmi_ctx->hcr & HCR_TGE)) \
+ write_sysreg(nmi_ctx->hcr, hcr_el2); \
+} while (0)

static inline void ack_bad_irq(unsigned int irq)
{