[lttng-dev] [RFC PATCH urcu] Fix: dynamic fallback to compat futex on sys_futex ENOSYS

[Paul E. McKenney]

On Fri, Sep 11, 2015 at 10:48:38AM -0400, Mathieu Desnoyers wrote:
> Some MIPS processors (e.g. Cavium Octeon II) dynamically check if the
> CPU supports ll/sc within sys_futex, and return a ENOSYS errno if they
> don't, even though the architecture implements sys_futex.
> 
> Handle this situation by always building the sys_futex compatibility
> layer, and fall-back on it if sys_futex return a ENOSYS errno. This is
> a tiny compat layer which adds very little space overhead.
> 
> This adds an unlikely branch on return from sys_futex, which should
> not be an issue performance-wise (we've already taken a system call).
> 
> Since this is a fall-back mode, don't try to be clever, and don't cache
> the result, so that the common cases (architectures with a properly
> working sys_futex) don't get two conditional branches, just one.

Looks like a reasonable approach to me.

Acked-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>

> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> CC: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
> CC: Michael Jeanson <mjeanson at efficios.com>
> CC: Jon Bernard <jbernard at debian.org>
> ---
>  Makefile.am  |  2 --
>  urcu/futex.h | 70 +++++++++++++++++++++++++++++++++++++++++++++++++-----------
>  2 files changed, 57 insertions(+), 15 deletions(-)
> 
> diff --git a/Makefile.am b/Makefile.am
> index 752510d..f9a8888 100644
> --- a/Makefile.am
> +++ b/Makefile.am
> @@ -41,9 +41,7 @@ else
>  COMPAT=
>  endif
> 
> -if COMPAT_FUTEX
>  COMPAT+=compat_futex.c
> -endif
> 
>  RCULFHASH = rculfhash.c rculfhash-mm-order.c rculfhash-mm-chunk.c \
>  		rculfhash-mm-mmap.c
> diff --git a/urcu/futex.h b/urcu/futex.h
> index 2be3bb6..13d2b1a 100644
> --- a/urcu/futex.h
> +++ b/urcu/futex.h
> @@ -47,22 +47,66 @@ extern "C" {
>   * (returns EINTR).
>   */
> 
> +extern int compat_futex_noasync(int32_t *uaddr, int op, int32_t val,
> +		const struct timespec *timeout, int32_t *uaddr2, int32_t val3);
> +extern int compat_futex_async(int32_t *uaddr, int op, int32_t val,
> +		const struct timespec *timeout, int32_t *uaddr2, int32_t val3);
> +
>  #ifdef CONFIG_RCU_HAVE_FUTEX
> +
> +#include <unistd.h>
> +#include <errno.h>
> +#include <urcu/compiler.h>
>  #include <urcu/syscall-compat.h>
> -#define futex(...)	syscall(__NR_futex, __VA_ARGS__)
> -#define futex_noasync(uaddr, op, val, timeout, uaddr2, val3)	\
> -		futex(uaddr, op, val, timeout, uaddr2, val3)
> -#define futex_async(uaddr, op, val, timeout, uaddr2, val3)	\
> -		futex(uaddr, op, val, timeout, uaddr2, val3)
> +
> +static inline int futex(int32_t *uaddr, int op, int32_t val,
> +		const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
> +{
> +	return syscall(__NR_futex, uaddr, op, val, timeout,
> +			uaddr2, val3);
> +}
> +
> +static inline int futex_noasync(int32_t *uaddr, int op, int32_t val,
> +		const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
> +{
> +	int ret;
> +
> +	ret = futex(uaddr, op, val, timeout, uaddr2, val3);
> +	if (caa_unlikely(ret < 0 && errno == ENOSYS)) {
> +		return compat_futex_noasync(uaddr, op, val, timeout,
> +				uaddr2, val3);
> +	}
> +	return ret;
> +
> +}
> +
> +static inline int futex_async(int32_t *uaddr, int op, int32_t val,
> +		const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
> +{
> +	int ret;
> +
> +	ret = futex(uaddr, op, val, timeout, uaddr2, val3);
> +	if (caa_unlikely(ret < 0 && errno == ENOSYS)) {
> +		return compat_futex_async(uaddr, op, val, timeout,
> +				uaddr2, val3);
> +	}
> +	return ret;
> +}
> +
>  #else
> -extern int compat_futex_noasync(int32_t *uaddr, int op, int32_t val,
> -	const struct timespec *timeout, int32_t *uaddr2, int32_t val3);
> -#define futex_noasync(uaddr, op, val, timeout, uaddr2, val3)	\
> -		compat_futex_noasync(uaddr, op, val, timeout, uaddr2, val3)
> -extern int compat_futex_async(int32_t *uaddr, int op, int32_t val,
> -	const struct timespec *timeout, int32_t *uaddr2, int32_t val3);
> -#define futex_async(uaddr, op, val, timeout, uaddr2, val3)	\
> -		compat_futex_async(uaddr, op, val, timeout, uaddr2, val3)
> +
> +static inline int futex_noasync(int32_t *uaddr, int op, int32_t val,
> +		const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
> +{
> +	return compat_futex_noasync(uaddr, op, val, timeout, uaddr2, val3);
> +}
> +
> +static inline int futex_async(int32_t *uaddr, int op, int32_t val,
> +		const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
> +{
> +	return compat_futex_async(uaddr, op, val, timeout, uaddr2, val3);
> +}
> +
>  #endif
> 
>  #ifdef __cplusplus 
> -- 
> 2.1.4
>