[PATCH v2] riscv: Enable interrupt during exception handling

[Nam Cao]

force_sig_fault() takes a spinlock, which is a sleeping lock with
CONFIG_PREEMPT_RT=y. However, exception handling calls force_sig_fault()
with interrupt disabled, causing a sleeping in atomic context warning.

This can be reproduced using userspace programs such as:
    int main() { asm ("ebreak"); }
or
    int main() { asm ("unimp"); }

There is no reason that interrupt must be disabled while handling
exceptions from userspace.

Enable interrupt while handling user exceptions. This also has the added
benefit of avoiding unnecessary delays in interrupt handling.

Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
Suggested-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
Signed-off-by: Nam Cao <namcao at linutronix.de>
---
v2: stop enabling interrupts for kernel exceptions. For exceptions treated
like NMI, it is wrong. For page faults, interrupts are already
(conditionally) enabled.
---
 arch/riscv/kernel/traps.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index 9c83848797a78..80230de167def 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -6,6 +6,7 @@
 #include <linux/cpu.h>
 #include <linux/kernel.h>
 #include <linux/init.h>
+#include <linux/irqflags.h>
 #include <linux/randomize_kstack.h>
 #include <linux/sched.h>
 #include <linux/sched/debug.h>
@@ -151,7 +152,9 @@ asmlinkage __visible __trap_section void name(struct pt_regs *regs)		\
 {										\
 	if (user_mode(regs)) {							\
 		irqentry_enter_from_user_mode(regs);				\
+		local_irq_enable();						\
 		do_trap_error(regs, signo, code, regs->epc, "Oops - " str);	\
+		local_irq_disable();						\
 		irqentry_exit_to_user_mode(regs);				\
 	} else {								\
 		irqentry_state_t state = irqentry_nmi_enter(regs);		\
@@ -173,17 +176,14 @@ asmlinkage __visible __trap_section void do_trap_insn_illegal(struct pt_regs *re
 
 	if (user_mode(regs)) {
 		irqentry_enter_from_user_mode(regs);
-
 		local_irq_enable();
 
 		handled = riscv_v_first_use_handler(regs);
-
-		local_irq_disable();
-
 		if (!handled)
 			do_trap_error(regs, SIGILL, ILL_ILLOPC, regs->epc,
 				      "Oops - illegal instruction");
 
+		local_irq_disable();
 		irqentry_exit_to_user_mode(regs);
 	} else {
 		irqentry_state_t state = irqentry_nmi_enter(regs);
@@ -308,9 +308,11 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
 {
 	if (user_mode(regs)) {
 		irqentry_enter_from_user_mode(regs);
+		local_irq_enable();
 
 		handle_break(regs);
 
+		local_irq_disable();
 		irqentry_exit_to_user_mode(regs);
 	} else {
 		irqentry_state_t state = irqentry_nmi_enter(regs);
-- 
2.39.5