ARC: syscall for userspace cmpxchg assist

Older ARC700 cores (ARC750 specifically) lack instructions to implement
atomic r-w-w. This is problematic for userspace libraries such as NPTL
which need atomic primitives. So enable them by providing kernel assist.
This is costly but really the only sane soluton (othern than tight
spinning using the otherwise availiable atomic exchange EX instruciton).

Good thing is there are only a few of these cores running Linux out in
the wild.

This only works on UP systems.

Reviewed-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

diff --git a/arch/arc/include/asm/syscalls.h b/arch/arc/include/asm/syscalls.h
index e56f9fcc558133277ca03d93461a56da5f02a4b9..772b67ca56e7bacb307fe5f5a944b1318b188b97 100644 (file)
--- a/arch/arc/include/asm/syscalls.h
+++ b/arch/arc/include/asm/syscalls.h
@@ -17,6 +17,7 @@ int sys_clone_wrapper(int, int, int, int, int);
 int sys_cacheflush(uint32_t, uint32_t uint32_t);
 int sys_arc_settls(void *);
 int sys_arc_gettls(void);
+int sys_arc_usr_cmpxchg(int *, int, int);
 
 #include <asm-generic/syscalls.h>
 

diff --git a/arch/arc/include/uapi/asm/unistd.h b/arch/arc/include/uapi/asm/unistd.h
index 41fa2ec9e02c7721717e5c513bc9703ebed5bed4..9a34136d84b2c77b45ee3b3b7a739f2d994151d4 100644 (file)
--- a/arch/arc/include/uapi/asm/unistd.h
+++ b/arch/arc/include/uapi/asm/unistd.h
@@ -27,18 +27,19 @@
 
 #define NR_syscalls    __NR_syscalls
 
+/* Generic syscall (fs/filesystems.c - lost in asm-generic/unistd.h */
+#define __NR_sysfs             (__NR_arch_specific_syscall + 3)
+
 /* ARC specific syscall */
 #define __NR_cacheflush                (__NR_arch_specific_syscall + 0)
 #define __NR_arc_settls                (__NR_arch_specific_syscall + 1)
 #define __NR_arc_gettls                (__NR_arch_specific_syscall + 2)
+#define __NR_arc_usr_cmpxchg   (__NR_arch_specific_syscall + 4)
 
 __SYSCALL(__NR_cacheflush, sys_cacheflush)
 __SYSCALL(__NR_arc_settls, sys_arc_settls)
 __SYSCALL(__NR_arc_gettls, sys_arc_gettls)
-
-
-/* Generic syscall (fs/filesystems.c - lost in asm-generic/unistd.h */
-#define __NR_sysfs             (__NR_arch_specific_syscall + 3)
+__SYSCALL(__NR_arc_usr_cmpxchg, sys_arc_usr_cmpxchg)
 __SYSCALL(__NR_sysfs, sys_sysfs)
 
 #undef __SYSCALL

diff --git a/arch/arc/kernel/process.c b/arch/arc/kernel/process.c
index be1972bd2729e7a41013d521e9349cd4ac028499..59aa43cb146e732e01544790913f0b018a923cc6 100644 (file)
--- a/arch/arc/kernel/process.c
+++ b/arch/arc/kernel/process.c
@@ -41,6 +41,39 @@ SYSCALL_DEFINE0(arc_gettls)
        return task_thread_info(current)->thr_ptr;
 }
 
+SYSCALL_DEFINE3(arc_usr_cmpxchg, int *, uaddr, int, expected, int, new)
+{
+       int uval;
+       int ret;
+
+       /*
+        * This is only for old cores lacking LLOCK/SCOND, which by defintion
+        * can't possibly be SMP. Thus doesn't need to be SMP safe.
+        * And this also helps reduce the overhead for serializing in
+        * the UP case
+        */
+       WARN_ON_ONCE(IS_ENABLED(CONFIG_SMP));
+
+       if (!access_ok(VERIFY_WRITE, uaddr, sizeof(int)))
+               return -EFAULT;
+
+       preempt_disable();
+
+       ret = __get_user(uval, uaddr);
+       if (ret)
+               goto done;
+
+       if (uval != expected)
+               ret = -EAGAIN;
+       else
+               ret = __put_user(new, uaddr);
+
+done:
+       preempt_enable();
+
+       return ret;
+}
+
 void arch_cpu_idle(void)
 {
        /* sleep, but enable all interrupts before committing */