On 10/24/2016 09:17 AM, Vineet Gupta wrote:
> 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 avaialble 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 at canonical.com>
> Signed-off-by: Vineet Gupta <vgupta at synopsys.com>
> ---
> Changes since v1
> - errno not returned for access_ok() failing [Colin]
> - Beefed up change log
> - WARN_ON_ONCE() for CONFIG_SMP since this is only UP safe
> ---
> arch/arc/include/asm/syscalls.h | 1 +
> arch/arc/include/uapi/asm/unistd.h | 9 +++++----
> arch/arc/kernel/process.c | 33 +++++++++++++++++++++++++++++++++
> 3 files changed, 39 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arc/include/asm/syscalls.h b/arch/arc/include/asm/syscalls.h
> index e56f9fcc5581..772b67ca56e7 100644
> --- 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 41fa2ec9e02c..9a34136d84b2 100644
> --- 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 be1972bd2729..59aa43cb146e 100644
> --- 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;
> +}
It seems there is a subtle issue with this interface. Userspace cares more about
"prev" value to be able to build it's own state machine(s) - my existing uclibc
code was flawed as it was tight looping on the errno result.
We can add a return param, by passing a pointer, but I think it would be better
(and slightly cheaper) to just ditch the errno and simply return the prev value
which and current value could be checked for success/fail decision etc.
-Vineet
