[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>

3 files changed, 68 insertions, 5 deletions

diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index 4eb36155dc93..e14278d59882 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/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:
diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
index 14df16b983f4..45d2a032d890 100644
--- a/arch/arm/kernel/traps.c
+++ b/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
diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index ade0e2222f59..3fefb43c67f7 100644
--- a/arch/arm/mm/Kconfig
+++ b/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.
+