[PATCH] ARM: 2664/2: add support for atomic ops on pre-ARMv6 SMP systems

Patch from Nicolas Pitre

Not that there might be many of them on the planet, but at least RMK
apparently has one.

Signed-off-by: Nicolas Pitre
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/entry-armv.S

@@ -269,7 +269,7 @@ __pabt_svc:
 	add	r5, sp, #S_PC
 	ldmia	r7, {r2 - r4}			@ Get USR pc, cpsr
 
-#if __LINUX_ARM_ARCH__ < 6
+#if __LINUX_ARM_ARCH__ < 6 && !defined(CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG)
 	@ make sure our user space atomic helper is aborted
 	cmp	r2, #VIRT_OFFSET
 	bichs	r3, r3, #PSR_Z_BIT
@@ -616,11 +616,17 @@ __kuser_helper_start:
 
 __kuser_cmpxchg:				@ 0xffff0fc0
 
-#if __LINUX_ARM_ARCH__ < 6
+#if defined(CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG)
 
-#ifdef CONFIG_SMP  /* sanity check */
-#error "CONFIG_SMP on a machine supporting pre-ARMv6 processors?"
-#endif
+	/*
+	 * Poor you.  No fast solution possible...
+	 * The kernel itself must perform the operation.
+	 * A special ghost syscall is used for that (see traps.c).
+	 */
+	swi	#0x9ffff0
+	mov	pc, lr
+
+#elif __LINUX_ARM_ARCH__ < 6
 
 	/*
 	 * Theory of operation:

arch/arm/kernel/traps.c

@@ -464,6 +464,55 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs)
 #endif
 		return 0;
 
+#ifdef CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG
+	/*
+	 * Atomically store r1 in *r2 if *r2 is equal to r0 for user space.
+	 * Return zero in r0 if *MEM was changed or non-zero if no exchange
+	 * happened.  Also set the user C flag accordingly.
+	 * If access permissions have to be fixed up then non-zero is
+	 * returned and the operation has to be re-attempted.
+	 *
+	 * *NOTE*: This is a ghost syscall private to the kernel.  Only the
+	 * __kuser_cmpxchg code in entry-armv.S should be aware of its
+	 * existence.  Don't ever use this from user code.
+	 */
+	case 0xfff0:
+	{
+		extern void do_DataAbort(unsigned long addr, unsigned int fsr,
+					 struct pt_regs *regs);
+		unsigned long val;
+		unsigned long addr = regs->ARM_r2;
+		struct mm_struct *mm = current->mm;
+		pgd_t *pgd; pmd_t *pmd; pte_t *pte;
+
+		regs->ARM_cpsr &= ~PSR_C_BIT;
+		spin_lock(&mm->page_table_lock);
+		pgd = pgd_offset(mm, addr);
+		if (!pgd_present(*pgd))
+			goto bad_access;
+		pmd = pmd_offset(pgd, addr);
+		if (!pmd_present(*pmd))
+			goto bad_access;
+		pte = pte_offset_map(pmd, addr);
+		if (!pte_present(*pte) || !pte_write(*pte))
+			goto bad_access;
+		val = *(unsigned long *)addr;
+		val -= regs->ARM_r0;
+		if (val == 0) {
+			*(unsigned long *)addr = regs->ARM_r1;
+			regs->ARM_cpsr |= PSR_C_BIT;
+		}
+		spin_unlock(&mm->page_table_lock);
+		return val;
+
+		bad_access:
+		spin_unlock(&mm->page_table_lock);
+		/* simulate a read access fault */
+		do_DataAbort(addr, 15 + (1 << 11), regs);
+		return -1;
+	}
+#endif
+
 	default:
 		/* Calls 9f00xx..9f07ff are defined to return -ENOSYS
 		   if not implemented, rather than raising SIGILL.  This

arch/arm/mm/Kconfig

@@ -422,3 +422,11 @@ config HAS_TLS_REG
 	  assume directly accessing that register and always obtain the
 	  expected value only on ARMv7 and above.
 
+config NEEDS_SYSCALL_FOR_CMPXCHG
+	bool
+	default y if SMP && (CPU_32v5 || CPU_32v4 || CPU_32v3)
+	help
+	  SMP on a pre-ARMv6 processor?  Well OK then.
+	  Forget about fast user space cmpxchg support.
+	  It is just not possible.
+