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Subject[PATCH v13 07/12] arm64: factor work_pending state machine to C
Date
Currently ret_fast_syscall, work_pending, and ret_to_user form an ad-hoc
state machine that can be difficult to reason about due to duplicated
code and a large number of branch targets.

This patch factors the common logic out into the existing
do_notify_resume function, converting the code to C in the process,
making the code more legible.

This patch tries to closely mirror the existing behaviour while using
the usual C control flow primitives. As local_irq_{disable,enable} may
be instrumented, we balance exception entry (where we will almost most
likely enable IRQs) with a call to trace_hardirqs_on just before the
return to userspace.

Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
---
arch/arm64/kernel/entry.S | 12 ++++--------
arch/arm64/kernel/signal.c | 36 ++++++++++++++++++++++++++----------
2 files changed, 30 insertions(+), 18 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 6c3b7345a6c4..2bbf7753c674 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -689,18 +689,13 @@ ret_fast_syscall_trace:
* Ok, we need to do extra processing, enter the slow path.
*/
work_pending:
- tbnz x1, #TIF_NEED_RESCHED, work_resched
- /* TIF_SIGPENDING, TIF_NOTIFY_RESUME or TIF_FOREIGN_FPSTATE case */
mov x0, sp // 'regs'
- enable_irq // enable interrupts for do_notify_resume()
bl do_notify_resume
- b ret_to_user
-work_resched:
#ifdef CONFIG_TRACE_IRQFLAGS
- bl trace_hardirqs_off // the IRQs are off here, inform the tracing code
+ bl trace_hardirqs_on // enabled while in userspace
#endif
- bl schedule
-
+ ldr x1, [tsk, #TI_FLAGS] // re-check for single-step
+ b finish_ret_to_user
/*
* "slow" syscall return path.
*/
@@ -709,6 +704,7 @@ ret_to_user:
ldr x1, [tsk, #TI_FLAGS]
and x2, x1, #_TIF_WORK_MASK
cbnz x2, work_pending
+finish_ret_to_user:
enable_step_tsk x1, x2
kernel_exit 0
ENDPROC(ret_to_user)
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index a8eafdbc7cb8..404dd67080b9 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -402,15 +402,31 @@ static void do_signal(struct pt_regs *regs)
asmlinkage void do_notify_resume(struct pt_regs *regs,
unsigned int thread_flags)
{
- if (thread_flags & _TIF_SIGPENDING)
- do_signal(regs);
-
- if (thread_flags & _TIF_NOTIFY_RESUME) {
- clear_thread_flag(TIF_NOTIFY_RESUME);
- tracehook_notify_resume(regs);
- }
-
- if (thread_flags & _TIF_FOREIGN_FPSTATE)
- fpsimd_restore_current_state();
+ /*
+ * The assembly code enters us with IRQs off, but it hasn't
+ * informed the tracing code of that for efficiency reasons.
+ * Update the trace code with the current status.
+ */
+ trace_hardirqs_off();
+ do {
+ if (thread_flags & _TIF_NEED_RESCHED) {
+ schedule();
+ } else {
+ local_irq_enable();
+
+ if (thread_flags & _TIF_SIGPENDING)
+ do_signal(regs);
+
+ if (thread_flags & _TIF_NOTIFY_RESUME) {
+ clear_thread_flag(TIF_NOTIFY_RESUME);
+ tracehook_notify_resume(regs);
+ }
+
+ if (thread_flags & _TIF_FOREIGN_FPSTATE)
+ fpsimd_restore_current_state();
+ }

+ local_irq_disable();
+ thread_flags = READ_ONCE(current_thread_info()->flags);
+ } while (thread_flags & _TIF_WORK_MASK);
}
--
2.7.2
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