Merge tag 'core-rcu-2020-06-01' of git://git.kernel.org/pub/scm/linux…

…/kernel/git/tip/tip

Pull RCU updates from Ingo Molnar:
 "The RCU updates for this cycle were:

   - RCU-tasks update, including addition of RCU Tasks Trace for BPF use
     and TASKS_RUDE_RCU

   - kfree_rcu() updates.

   - Remove scheduler locking restriction

   - RCU CPU stall warning updates.

   - Torture-test updates.

   - Miscellaneous fixes and other updates"

* tag 'core-rcu-2020-06-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (103 commits)
  rcu: Allow for smp_call_function() running callbacks from idle
  rcu: Provide rcu_irq_exit_check_preempt()
  rcu: Abstract out rcu_irq_enter_check_tick() from rcu_nmi_enter()
  rcu: Provide __rcu_is_watching()
  rcu: Provide rcu_irq_exit_preempt()
  rcu: Make RCU IRQ enter/exit functions rely on in_nmi()
  rcu/tree: Mark the idle relevant functions noinstr
  x86: Replace ist_enter() with nmi_enter()
  x86/mce: Send #MC singal from task work
  x86/entry: Get rid of ist_begin/end_non_atomic()
  sched,rcu,tracing: Avoid tracing before in_nmi() is correct
  sh/ftrace: Move arch_ftrace_nmi_{enter,exit} into nmi exception
  lockdep: Always inline lockdep_{off,on}()
  hardirq/nmi: Allow nested nmi_enter()
  arm64: Prepare arch_nmi_enter() for recursion
  printk: Disallow instrumenting print_nmi_enter()
  printk: Prepare for nested printk_nmi_enter()
  rcutorture: Convert ULONG_CMP_LT() to time_before()
  torture: Add a --kasan argument
  torture: Save a few lines by using config_override_param initially
  ...

Documentation/RCU/Design/Requirements/Requirements.rst

@@ -1943,56 +1943,27 @@ invoked from a CPU-hotplug notifier.
 Scheduler and RCU
 ~~~~~~~~~~~~~~~~~
 
-RCU depends on the scheduler, and the scheduler uses RCU to protect some
-of its data structures. The preemptible-RCU ``rcu_read_unlock()``
-implementation must therefore be written carefully to avoid deadlocks
-involving the scheduler's runqueue and priority-inheritance locks. In
-particular, ``rcu_read_unlock()`` must tolerate an interrupt where the
-interrupt handler invokes both ``rcu_read_lock()`` and
-``rcu_read_unlock()``. This possibility requires ``rcu_read_unlock()``
-to use negative nesting levels to avoid destructive recursion via
-interrupt handler's use of RCU.
-
-This scheduler-RCU requirement came as a `complete
-surprise <https://lwn.net/Articles/453002/>`__.
-
-As noted above, RCU makes use of kthreads, and it is necessary to avoid
-excessive CPU-time accumulation by these kthreads. This requirement was
-no surprise, but RCU's violation of it when running context-switch-heavy
-workloads when built with ``CONFIG_NO_HZ_FULL=y`` `did come as a
-surprise
+RCU makes use of kthreads, and it is necessary to avoid excessive CPU-time
+accumulation by these kthreads. This requirement was no surprise, but
+RCU's violation of it when running context-switch-heavy workloads when
+built with ``CONFIG_NO_HZ_FULL=y`` `did come as a surprise
 [PDF] <http://www.rdrop.com/users/paulmck/scalability/paper/BareMetal.2015.01.15b.pdf>`__.
 RCU has made good progress towards meeting this requirement, even for
 context-switch-heavy ``CONFIG_NO_HZ_FULL=y`` workloads, but there is
 room for further improvement.
 
-It is forbidden to hold any of scheduler's runqueue or
-priority-inheritance spinlocks across an ``rcu_read_unlock()`` unless
-interrupts have been disabled across the entire RCU read-side critical
-section, that is, up to and including the matching ``rcu_read_lock()``.
-Violating this restriction can result in deadlocks involving these
-scheduler spinlocks. There was hope that this restriction might be
-lifted when interrupt-disabled calls to ``rcu_read_unlock()`` started
-deferring the reporting of the resulting RCU-preempt quiescent state
-until the end of the corresponding interrupts-disabled region.
-Unfortunately, timely reporting of the corresponding quiescent state to
-expedited grace periods requires a call to ``raise_softirq()``, which
-can acquire these scheduler spinlocks. In addition, real-time systems
-using RCU priority boosting need this restriction to remain in effect
-because deferred quiescent-state reporting would also defer deboosting,
-which in turn would degrade real-time latencies.
-
-In theory, if a given RCU read-side critical section could be guaranteed
-to be less than one second in duration, holding a scheduler spinlock
-across that critical section's ``rcu_read_unlock()`` would require only
-that preemption be disabled across the entire RCU read-side critical
-section, not interrupts. Unfortunately, given the possibility of vCPU
-preemption, long-running interrupts, and so on, it is not possible in
-practice to guarantee that a given RCU read-side critical section will
-complete in less than one second. Therefore, as noted above, if
-scheduler spinlocks are held across a given call to
-``rcu_read_unlock()``, interrupts must be disabled across the entire RCU
-read-side critical section.
+There is no longer any prohibition against holding any of
+scheduler's runqueue or priority-inheritance spinlocks across an
+``rcu_read_unlock()``, even if interrupts and preemption were enabled
+somewhere within the corresponding RCU read-side critical section.
+Therefore, it is now perfectly legal to execute ``rcu_read_lock()``
+with preemption enabled, acquire one of the scheduler locks, and hold
+that lock across the matching ``rcu_read_unlock()``.
+
+Similarly, the RCU flavor consolidation has removed the need for negative
+nesting.  The fact that interrupt-disabled regions of code act as RCU
+read-side critical sections implicitly avoids earlier issues that used
+to result in destructive recursion via interrupt handler's use of RCU.
 
 Tracing and RCU
 ~~~~~~~~~~~~~~~

Documentation/admin-guide/kernel-parameters.txt

@@ -4210,12 +4210,24 @@
 			Duration of CPU stall (s) to test RCU CPU stall
 			warnings, zero to disable.
 
+	rcutorture.stall_cpu_block= [KNL]
+			Sleep while stalling if set.  This will result
+			in warnings from preemptible RCU in addition
+			to any other stall-related activity.
+
 	rcutorture.stall_cpu_holdoff= [KNL]
 			Time to wait (s) after boot before inducing stall.
 
 	rcutorture.stall_cpu_irqsoff= [KNL]
 			Disable interrupts while stalling if set.
 
+	rcutorture.stall_gp_kthread= [KNL]
+			Duration (s) of forced sleep within RCU
+			grace-period kthread to test RCU CPU stall
+			warnings, zero to disable.  If both stall_cpu
+			and stall_gp_kthread are specified, the
+			kthread is starved first, then the CPU.
+
 	rcutorture.stat_interval= [KNL]
 			Time (s) between statistics printk()s.
 
@@ -4286,6 +4298,13 @@
 			only normal grace-period primitives.  No effect
 			on CONFIG_TINY_RCU kernels.
 
+	rcupdate.rcu_task_ipi_delay= [KNL]
+			Set time in jiffies during which RCU tasks will
+			avoid sending IPIs, starting with the beginning
+			of a given grace period.  Setting a large
+			number avoids disturbing real-time workloads,
+			but lengthens grace periods.
+
 	rcupdate.rcu_task_stall_timeout= [KNL]
 			Set timeout in jiffies for RCU task stall warning
 			messages.  Disable with a value less than or equal

Documentation/trace/ftrace-design.rst

@@ -229,14 +229,6 @@ Adding support for it is easy: just define the macro in asm/ftrace.h and
 pass the return address pointer as the 'retp' argument to
 ftrace_push_return_trace().
 
-HAVE_FTRACE_NMI_ENTER
----------------------
-
-If you can't trace NMI functions, then skip this option.
-
-<details to be filled>
-
-
 HAVE_SYSCALL_TRACEPOINTS
 ------------------------
 

arch/arm64/include/asm/hardirq.h

@@ -32,30 +32,70 @@ 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()) {					\
-			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;						\
+	u64 ___hcr;							\
+									\
+	if (!is_kernel_in_hyp_mode())					\
+		break;							\
+									\
+	___ctx = this_cpu_ptr(&nmi_contexts);				\
+	if (___ctx->cnt) {						\
+		___ctx->cnt++;						\
+		break;							\
+	}								\
+									\
+	___hcr = read_sysreg(hcr_el2);					\
+	if (!(___hcr & HCR_TGE)) {					\
+		write_sysreg(___hcr | HCR_TGE, hcr_el2);		\
+		isb();							\
+	}								\
+	/*								\
+	 * Make sure the sysreg write is performed before ___ctx->cnt	\
+	 * is set to 1. NMIs that see cnt == 1 will rely on us.		\
+	 */								\
+	barrier();							\
+	___ctx->cnt = 1;                                                \
+	/*								\
+	 * Make sure ___ctx->cnt is set before we save ___hcr. We	\
+	 * don't want ___ctx->hcr to be overwritten.			\
+	 */								\
+	barrier();							\
+	___ctx->hcr = ___hcr;						\
+} while (0)
 
-#define arch_nmi_exit()								\
-	do {									\
-		if (is_kernel_in_hyp_mode()) {					\
-			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;						\
+	u64 ___hcr;							\
+									\
+	if (!is_kernel_in_hyp_mode())					\
+		break;							\
+									\
+	___ctx = this_cpu_ptr(&nmi_contexts);				\
+	___hcr = ___ctx->hcr;						\
+	/*								\
+	 * Make sure we read ___ctx->hcr before we release		\
+	 * ___ctx->cnt as it makes ___ctx->hcr updatable again.		\
+	 */								\
+	barrier();							\
+	___ctx->cnt--;							\
+	/*								\
+	 * Make sure ___ctx->cnt release is visible before we		\
+	 * restore the sysreg. Otherwise a new NMI occurring		\
+	 * right after write_sysreg() can be fooled and think		\
+	 * we secured things for it.					\
+	 */								\
+	barrier();							\
+	if (!___ctx->cnt && !(___hcr & HCR_TGE))			\
+		write_sysreg(___hcr, hcr_el2);				\
+} while (0)
 
 static inline void ack_bad_irq(unsigned int irq)
 {

arch/arm64/kernel/sdei.c

@@ -251,22 +251,12 @@ asmlinkage __kprobes notrace unsigned long
 __sdei_handler(struct pt_regs *regs, struct sdei_registered_event *arg)
 {
 	unsigned long ret;
-	bool do_nmi_exit = false;
 
-	/*
-	 * nmi_enter() deals with printk() re-entrance and use of RCU when
-	 * RCU believed this CPU was idle. Because critical events can
-	 * interrupt normal events, we may already be in_nmi().
-	 */
-	if (!in_nmi()) {
-		nmi_enter();
-		do_nmi_exit = true;
-	}
+	nmi_enter();
 
 	ret = _sdei_handler(regs, arg);
 
-	if (do_nmi_exit)
-		nmi_exit();
+	nmi_exit();
 
 	return ret;
 }

arch/arm64/kernel/traps.c

@@ -906,17 +906,13 @@ bool arm64_is_fatal_ras_serror(struct pt_regs *regs, unsigned int esr)
 
 asmlinkage void do_serror(struct pt_regs *regs, unsigned int esr)
 {
-	const bool was_in_nmi = in_nmi();
-
-	if (!was_in_nmi)
-		nmi_enter();
+	nmi_enter();
 
 	/* non-RAS errors are not containable */
 	if (!arm64_is_ras_serror(esr) || arm64_is_fatal_ras_serror(regs, esr))
 		arm64_serror_panic(regs, esr);
 
-	if (!was_in_nmi)
-		nmi_exit();
+	nmi_exit();
 }
 
 asmlinkage void enter_from_user_mode(void)

arch/powerpc/kernel/traps.c

@@ -441,15 +441,9 @@ void hv_nmi_check_nonrecoverable(struct pt_regs *regs)
 void system_reset_exception(struct pt_regs *regs)
 {
 	unsigned long hsrr0, hsrr1;
-	bool nested = in_nmi();
 	bool saved_hsrrs = false;
 
-	/*
-	 * Avoid crashes in case of nested NMI exceptions. Recoverability
-	 * is determined by RI and in_nmi
-	 */
-	if (!nested)
-		nmi_enter();
+	nmi_enter();
 
 	/*
 	 * System reset can interrupt code where HSRRs are live and MSR[RI]=1.
@@ -521,8 +515,7 @@ void system_reset_exception(struct pt_regs *regs)
 		mtspr(SPRN_HSRR1, hsrr1);
 	}
 
-	if (!nested)
-		nmi_exit();
+	nmi_exit();
 
 	/* What should we do here? We could issue a shutdown or hard reset. */
 }
@@ -823,9 +816,8 @@ int machine_check_generic(struct pt_regs *regs)
 void machine_check_exception(struct pt_regs *regs)
 {
 	int recover = 0;
-	bool nested = in_nmi();
-	if (!nested)
-		nmi_enter();
+
+	nmi_enter();
 
 	__this_cpu_inc(irq_stat.mce_exceptions);
 
@@ -851,8 +843,7 @@ void machine_check_exception(struct pt_regs *regs)
 	if (check_io_access(regs))
 		goto bail;
 
-	if (!nested)
-		nmi_exit();
+	nmi_exit();
 
 	die("Machine check", regs, SIGBUS);
 
@@ -863,8 +854,7 @@ void machine_check_exception(struct pt_regs *regs)
 	return;
 
 bail:
-	if (!nested)
-		nmi_exit();
+	nmi_exit();
 }
 
 void SMIException(struct pt_regs *regs)

arch/sh/Kconfig

@@ -71,7 +71,6 @@ config SUPERH32
 	select HAVE_FUNCTION_TRACER
 	select HAVE_FTRACE_MCOUNT_RECORD
 	select HAVE_DYNAMIC_FTRACE
-	select HAVE_FTRACE_NMI_ENTER if DYNAMIC_FTRACE
 	select ARCH_WANT_IPC_PARSE_VERSION
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_ARCH_KGDB

arch/sh/kernel/traps.c

@@ -170,11 +170,21 @@ BUILD_TRAP_HANDLER(bug)
 	force_sig(SIGTRAP);
 }
 
+#ifdef CONFIG_DYNAMIC_FTRACE
+extern void arch_ftrace_nmi_enter(void);
+extern void arch_ftrace_nmi_exit(void);
+#else
+static inline void arch_ftrace_nmi_enter(void) { }
+static inline void arch_ftrace_nmi_exit(void) { }
+#endif
+
 BUILD_TRAP_HANDLER(nmi)
 {
 	unsigned int cpu = smp_processor_id();
 	TRAP_HANDLER_DECL;
 
+	arch_ftrace_nmi_enter();
+
 	nmi_enter();
 	nmi_count(cpu)++;
 
@@ -190,4 +200,6 @@ BUILD_TRAP_HANDLER(nmi)
 	}
 
 	nmi_exit();
+
+	arch_ftrace_nmi_exit();
 }

arch/x86/include/asm/traps.h

@@ -118,11 +118,6 @@ void smp_spurious_interrupt(struct pt_regs *regs);
 void smp_error_interrupt(struct pt_regs *regs);
 asmlinkage void smp_irq_move_cleanup_interrupt(void);
 
-extern void ist_enter(struct pt_regs *regs);
-extern void ist_exit(struct pt_regs *regs);
-extern void ist_begin_non_atomic(struct pt_regs *regs);
-extern void ist_end_non_atomic(void);
-
 #ifdef CONFIG_VMAP_STACK
 void __noreturn handle_stack_overflow(const char *message,
 				      struct pt_regs *regs,