arch/tile: adopt prepare_exit_to_usermode() model from x86

This change is a prerequisite change for TASK_ISOLATION but also
stands on its own for readability and maintainability.  The existing
tile do_work_pending() was called in a loop from assembly on
the slow path; this change moves the loop into C code as well.
For the x86 version see commit c5c46f59 ("x86/entry: Add new,
comprehensible entry and exit handlers written in C").

This change exposes a pre-existing bug on the older tilepro platform;
the singlestep processing is done last, but on tilepro (unlike tilegx)
we enable interrupts while doing that processing, so we could in
theory miss a signal or other asynchronous event.  A future change
could fix this by breaking the singlestep work into a "prepare"
step done in the main loop, and a "trigger" step done after exiting
the loop.  Since this change is intended as purely a restructuring
change, we call out the bug explicitly now, but don't yet fix it.
Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>

arch/tile/include/asm/processor.h

@@ -212,7 +212,7 @@ static inline void release_thread(struct task_struct *dead_task)
 	/* Nothing for now */
 }
 
-extern int do_work_pending(struct pt_regs *regs, u32 flags);
+extern void prepare_exit_to_usermode(struct pt_regs *regs, u32 flags);
 
 
 /*

arch/tile/include/asm/thread_info.h

@@ -140,10 +140,14 @@ extern void _cpu_idle(void);
 #define _TIF_POLLING_NRFLAG	(1<<TIF_POLLING_NRFLAG)
 #define _TIF_NOHZ		(1<<TIF_NOHZ)
 
+/* Work to do as we loop to exit to user space. */
+#define _TIF_WORK_MASK \
+	(_TIF_SIGPENDING | _TIF_NEED_RESCHED | \
+	 _TIF_ASYNC_TLB | _TIF_NOTIFY_RESUME)
+
 /* Work to do on any return to user space. */
 #define _TIF_ALLWORK_MASK \
-	(_TIF_SIGPENDING | _TIF_NEED_RESCHED | _TIF_SINGLESTEP | \
-	 _TIF_ASYNC_TLB | _TIF_NOTIFY_RESUME | _TIF_NOHZ)
+	(_TIF_WORK_MASK | _TIF_SINGLESTEP | _TIF_NOHZ)
 
 /* Work to do at syscall entry. */
 #define _TIF_SYSCALL_ENTRY_WORK \

arch/tile/kernel/intvec_32.S

@@ -845,18 +845,6 @@ STD_ENTRY(interrupt_return)
 .Lresume_userspace:
 	FEEDBACK_REENTER(interrupt_return)
 
-	/*
-	 * Use r33 to hold whether we have already loaded the callee-saves
-	 * into ptregs.  We don't want to do it twice in this loop, since
-	 * then we'd clobber whatever changes are made by ptrace, etc.
-	 * Get base of stack in r32.
-	 */
-	{
-	 GET_THREAD_INFO(r32)
-	 movei  r33, 0
-	}
-
-.Lretry_work_pending:
 	/*
 	 * Disable interrupts so as to make sure we don't
 	 * miss an interrupt that sets any of the thread flags (like
@@ -867,33 +855,27 @@ STD_ENTRY(interrupt_return)
 	IRQ_DISABLE(r20, r21)
 	TRACE_IRQS_OFF  /* Note: clobbers registers r0-r29 */
 
-
-	/* Check to see if there is any work to do before returning to user. */
+	/*
+	 * See if there are any work items (including single-shot items)
+	 * to do.  If so, save the callee-save registers to pt_regs
+	 * and then dispatch to C code.
+	 */
+	GET_THREAD_INFO(r21)
 	{
-	 addi   r29, r32, THREAD_INFO_FLAGS_OFFSET
-	 moveli r1, lo16(_TIF_ALLWORK_MASK)
+	 addi   r22, r21, THREAD_INFO_FLAGS_OFFSET
+	 moveli r20, lo16(_TIF_ALLWORK_MASK)
 	}
 	{
-	 lw     r29, r29
-	 auli   r1, r1, ha16(_TIF_ALLWORK_MASK)
+	 lw     r22, r22
+	 auli   r20, r20, ha16(_TIF_ALLWORK_MASK)
 	}
-	and     r1, r29, r1
-	bzt     r1, .Lrestore_all
-
-	/*
-	 * Make sure we have all the registers saved for signal
-	 * handling, notify-resume, or single-step.  Call out to C
-	 * code to figure out exactly what we need to do for each flag bit,
-	 * then if necessary, reload the flags and recheck.
-	 */
+	and     r1, r22, r20
 	{
 	 PTREGS_PTR(r0, PTREGS_OFFSET_BASE)
-	 bnz    r33, 1f
+	 bzt    r1, .Lrestore_all
 	}
 	push_extra_callee_saves r0
-	movei   r33, 1
-1:	jal     do_work_pending
-	bnz     r0, .Lretry_work_pending
+	jal     prepare_exit_to_usermode
 
 	/*
 	 * In the NMI case we
@@ -1327,7 +1309,7 @@ STD_ENTRY(ret_from_kernel_thread)
 	FEEDBACK_REENTER(ret_from_kernel_thread)
 	{
 	 movei  r30, 0               /* not an NMI */
-	 j      .Lresume_userspace   /* jump into middle of interrupt_return */
+	 j      interrupt_return
 	}
 	STD_ENDPROC(ret_from_kernel_thread)
 

arch/tile/kernel/intvec_64.S

@@ -878,20 +878,6 @@ STD_ENTRY(interrupt_return)
 .Lresume_userspace:
 	FEEDBACK_REENTER(interrupt_return)
 
-	/*
-	 * Use r33 to hold whether we have already loaded the callee-saves
-	 * into ptregs.  We don't want to do it twice in this loop, since
-	 * then we'd clobber whatever changes are made by ptrace, etc.
-	 */
-	{
-	 movei  r33, 0
-	 move   r32, sp
-	}
-
-	/* Get base of stack in r32. */
-	EXTRACT_THREAD_INFO(r32)
-
-.Lretry_work_pending:
 	/*
 	 * Disable interrupts so as to make sure we don't
 	 * miss an interrupt that sets any of the thread flags (like
@@ -902,33 +888,28 @@ STD_ENTRY(interrupt_return)
 	IRQ_DISABLE(r20, r21)
 	TRACE_IRQS_OFF  /* Note: clobbers registers r0-r29 */
 
-
-	/* Check to see if there is any work to do before returning to user. */
+	/*
+	 * See if there are any work items (including single-shot items)
+	 * to do.  If so, save the callee-save registers to pt_regs
+	 * and then dispatch to C code.
+	 */
+	move    r21, sp
+	EXTRACT_THREAD_INFO(r21)
 	{
-	 addi   r29, r32, THREAD_INFO_FLAGS_OFFSET
-	 moveli r1, hw1_last(_TIF_ALLWORK_MASK)
+	 addi   r22, r21, THREAD_INFO_FLAGS_OFFSET
+	 moveli r20, hw1_last(_TIF_ALLWORK_MASK)
 	}
 	{
-	 ld     r29, r29
-	 shl16insli r1, r1, hw0(_TIF_ALLWORK_MASK)
+	 ld     r22, r22
+	 shl16insli r20, r20, hw0(_TIF_ALLWORK_MASK)
 	}
-	and     r1, r29, r1
-	beqzt   r1, .Lrestore_all
-
-	/*
-	 * Make sure we have all the registers saved for signal
-	 * handling or notify-resume.  Call out to C code to figure out
-	 * exactly what we need to do for each flag bit, then if
-	 * necessary, reload the flags and recheck.
-	 */
+	and     r1, r22, r20
 	{
 	 PTREGS_PTR(r0, PTREGS_OFFSET_BASE)
-	 bnez   r33, 1f
+	 beqzt  r1, .Lrestore_all
 	}
 	push_extra_callee_saves r0
-	movei   r33, 1
-1:	jal     do_work_pending
-	bnez    r0, .Lretry_work_pending
+	jal     prepare_exit_to_usermode
 
 	/*
 	 * In the NMI case we
@@ -1411,7 +1392,7 @@ STD_ENTRY(ret_from_kernel_thread)
 	FEEDBACK_REENTER(ret_from_kernel_thread)
 	{
 	 movei  r30, 0               /* not an NMI */
-	 j      .Lresume_userspace   /* jump into middle of interrupt_return */
+	 j      interrupt_return
 	}
 	STD_ENDPROC(ret_from_kernel_thread)
 

arch/tile/kernel/process.c

@@ -462,54 +462,57 @@ struct task_struct *__sched _switch_to(struct task_struct *prev,
 
 /*
  * This routine is called on return from interrupt if any of the
- * TIF_WORK_MASK flags are set in thread_info->flags.  It is
- * entered with interrupts disabled so we don't miss an event
- * that modified the thread_info flags.  If any flag is set, we
- * handle it and return, and the calling assembly code will
- * re-disable interrupts, reload the thread flags, and call back
- * if more flags need to be handled.
- *
- * We return whether we need to check the thread_info flags again
- * or not.  Note that we don't clear TIF_SINGLESTEP here, so it's
- * important that it be tested last, and then claim that we don't
- * need to recheck the flags.
+ * TIF_ALLWORK_MASK flags are set in thread_info->flags.  It is
+ * entered with interrupts disabled so we don't miss an event that
+ * modified the thread_info flags.  We loop until all the tested flags
+ * are clear.  Note that the function is called on certain conditions
+ * that are not listed in the loop condition here (e.g. SINGLESTEP)
+ * which guarantees we will do those things once, and redo them if any
+ * of the other work items is re-done, but won't continue looping if
+ * all the other work is done.
  */
-int do_work_pending(struct pt_regs *regs, u32 thread_info_flags)
+void prepare_exit_to_usermode(struct pt_regs *regs, u32 thread_info_flags)
 {
-	/* If we enter in kernel mode, do nothing and exit the caller loop. */
-	if (!user_mode(regs))
-		return 0;
-
-	user_exit();
+	if (WARN_ON(!user_mode(regs)))
+		return;
 
-	/* Enable interrupts; they are disabled again on return to caller. */
+	do {
 		local_irq_enable();
 
-	if (thread_info_flags & _TIF_NEED_RESCHED) {
+		if (thread_info_flags & _TIF_NEED_RESCHED)
 			schedule();
-		return 1;
-	}
+
 #if CHIP_HAS_TILE_DMA()
-	if (thread_info_flags & _TIF_ASYNC_TLB) {
+		if (thread_info_flags & _TIF_ASYNC_TLB)
 			do_async_page_fault(regs);
-		return 1;
-	}
 #endif
-	if (thread_info_flags & _TIF_SIGPENDING) {
+
+		if (thread_info_flags & _TIF_SIGPENDING)
 			do_signal(regs);
-		return 1;
-	}
+
 		if (thread_info_flags & _TIF_NOTIFY_RESUME) {
 			clear_thread_flag(TIF_NOTIFY_RESUME);
 			tracehook_notify_resume(regs);
-		return 1;
 		}
-	if (thread_info_flags & _TIF_SINGLESTEP)
+
+		local_irq_disable();
+		thread_info_flags = READ_ONCE(current_thread_info()->flags);
+
+	} while (thread_info_flags & _TIF_WORK_MASK);
+
+	if (thread_info_flags & _TIF_SINGLESTEP) {
 		single_step_once(regs);
+#ifndef __tilegx__
+		/*
+		 * FIXME: on tilepro, since we enable interrupts in
+		 * this routine, it's possible that we miss a signal
+		 * or other asynchronous event.
+		 */
+		local_irq_disable();
+#endif
+	}
 
 	user_enter();
-
-	return 0;
 }
 
 unsigned long get_wchan(struct task_struct *p)