Re: [PATCH v2] linux: Avoid shifting a negative signed on POSIX timer interface

[Arjun Shankar <arjun@redhat.com>]

Hi Adhemerval,

> The current macros uses pid as signed value, which triggers a compiler
> warning for process and thread timers.  Replace MAKE_PROCESS_CPUCLOCK
> with static inline function that expects the pid as unsigned.  These
> are similar to what Linux does internally.
>
> Checked on x86_64-linux-gnu.

I tried building master with -Wshift-negative-value and ran into a
couple of other left-shifts on negative values in
sysdeps/x86/dl-cacheinfo.h:

> count_mask = ~(-1 << (count_mask + 1));

Perhaps a cast of the -1 to unsigned here would make the intent even
more clear, and get glibc building cleanly with
-Wshift-negative-value. What do you think?

Anyway, this patch itself looks good to me.

Reviewed-by: Arjun Shankar <arjun@redhat.com>

>  nptl/pthread_getcpuclockid.c                  |  2 +-
>  sysdeps/unix/sysv/linux/clock_getcpuclockid.c |  2 +-
>  sysdeps/unix/sysv/linux/clock_nanosleep.c     |  2 +-
>  .../unix/sysv/linux/kernel-posix-cpu-timers.h | 28 +++++++++++++++----
>  sysdeps/unix/sysv/linux/timer_create.c        |  4 +--
>  5 files changed, 28 insertions(+), 10 deletions(-)
>
> diff --git a/nptl/pthread_getcpuclockid.c b/nptl/pthread_getcpuclockid.c
> index 344bd6560e..b8bf09f550 100644
> --- a/nptl/pthread_getcpuclockid.c
> +++ b/nptl/pthread_getcpuclockid.c
> @@ -35,7 +35,7 @@ __pthread_getcpuclockid (pthread_t threadid, clockid_t *clockid)
>
>    /* The clockid_t value is a simple computation from the TID.  */
>
> -  const clockid_t tidclock = MAKE_THREAD_CPUCLOCK (pd->tid, CPUCLOCK_SCHED);
> +  const clockid_t tidclock = make_thread_cpuclock (pd->tid, CPUCLOCK_SCHED);
>
>    *clockid = tidclock;
>    return 0;

OK. Use the new function instead of the old macro.

> diff --git a/sysdeps/unix/sysv/linux/clock_getcpuclockid.c b/sysdeps/unix/sysv/linux/clock_getcpuclockid.c
> index 5534127ed7..355d3c86af 100644
> --- a/sysdeps/unix/sysv/linux/clock_getcpuclockid.c
> +++ b/sysdeps/unix/sysv/linux/clock_getcpuclockid.c
> @@ -29,7 +29,7 @@ __clock_getcpuclockid (pid_t pid, clockid_t *clock_id)
>    /* The clockid_t value is a simple computation from the PID.
>       But we do a clock_getres call to validate it.  */
>
> -  const clockid_t pidclock = MAKE_PROCESS_CPUCLOCK (pid, CPUCLOCK_SCHED);
> +  const clockid_t pidclock = make_process_cpuclock (pid, CPUCLOCK_SCHED);
>
>  #ifndef __NR_clock_getres_time64
>  # define __NR_clock_getres_time64 __NR_clock_getres

OK. Same.

> diff --git a/sysdeps/unix/sysv/linux/clock_nanosleep.c b/sysdeps/unix/sysv/linux/clock_nanosleep.c
> index befe6ecb8c..e610fd4e8d 100644
> --- a/sysdeps/unix/sysv/linux/clock_nanosleep.c
> +++ b/sysdeps/unix/sysv/linux/clock_nanosleep.c
> @@ -34,7 +34,7 @@ __clock_nanosleep_time64 (clockid_t clock_id, int flags,
>    if (clock_id == CLOCK_THREAD_CPUTIME_ID)
>      return EINVAL;
>    if (clock_id == CLOCK_PROCESS_CPUTIME_ID)
> -    clock_id = MAKE_PROCESS_CPUCLOCK (0, CPUCLOCK_SCHED);
> +    clock_id = PROCESS_CLOCK;
>
>    /* If the call is interrupted by a signal handler or encounters an error,
>       it returns a positive value similar to errno.  */

OK. PROCESS_CLOCK expands to the appropriate call to make_process_cpuclock.

> diff --git a/sysdeps/unix/sysv/linux/kernel-posix-cpu-timers.h b/sysdeps/unix/sysv/linux/kernel-posix-cpu-timers.h
> index 164a90ddeb..bea1e0e62d 100644
> --- a/sysdeps/unix/sysv/linux/kernel-posix-cpu-timers.h
> +++ b/sysdeps/unix/sysv/linux/kernel-posix-cpu-timers.h
> @@ -1,4 +1,12 @@
> -/* Parameters for the Linux kernel ABI for CPU clocks.  */
> +/*
> +  Parameters for the Linux kernel ABI for CPU clocks, the bit fields within
> +  a clockid:
> +
> +  - The most significant 29 bits hold either a pid or a file descriptor.
> +  - Bit 2 indicates whether a cpu clock refers to a thread or a process.
> +  - Bits 1 and 0 give the type: PROF=0, VIRT=1, SCHED=2, or FD=3.
> +  - A clockid is invalid if bits 2, 1, and 0 are all set.
> + */
>
>  #define CPUCLOCK_PID(clock)            ((pid_t) ~((clock) >> 3))
>  #define CPUCLOCK_PERTHREAD(clock) \

OK. Comment lines up with the one in Linux's "include/linux/posix-timers.h".

> @@ -12,7 +20,17 @@
>  #define CPUCLOCK_SCHED         2
>  #define CPUCLOCK_MAX           3
>
> -#define MAKE_PROCESS_CPUCLOCK(pid, clock) \
> -       ((~(clockid_t) (pid) << 3) | (clockid_t) (clock))

OK. clockid_t is a signed type, thus expanding with pid = 0 leads to a
left-shift on a negative value.

> -#define MAKE_THREAD_CPUCLOCK(tid, clock) \
> -       MAKE_PROCESS_CPUCLOCK((tid), (clock) | CPUCLOCK_PERTHREAD_MASK)

OK. Same macro used. Same issue.

> +static inline clockid_t
> +make_process_cpuclock (unsigned int pid, clockid_t clock)
> +{
> +  return ((~pid) << 3) | clock;
> +}
> +

OK. Replacement function that uses unsigned. Lines up with the version
in Linux's sources.

> +static inline clockid_t
> +make_thread_cpuclock (unsigned int tid, clockid_t clock)
> +{
> +  return make_process_cpuclock (tid, clock | CPUCLOCK_PERTHREAD_MASK);
> +}
> +

OK. Replaces the second macro for tid.

> +#define PROCESS_CLOCK  make_process_cpuclock (0, CPUCLOCK_SCHED)
> +#define THREAD_CLOCK   make_thread_cpuclock (0, CPUCLOCK_SCHED)

OK. Macros for convenience.

> diff --git a/sysdeps/unix/sysv/linux/timer_create.c b/sysdeps/unix/sysv/linux/timer_create.c
> index a8b2a41d9e..290324a7ea 100644
> --- a/sysdeps/unix/sysv/linux/timer_create.c
> +++ b/sysdeps/unix/sysv/linux/timer_create.c
> @@ -33,9 +33,9 @@ ___timer_create (clockid_t clock_id, struct sigevent *evp, timer_t *timerid)
>  {
>    {
>      clockid_t syscall_clockid = (clock_id == CLOCK_PROCESS_CPUTIME_ID
> -                                ? MAKE_PROCESS_CPUCLOCK (0, CPUCLOCK_SCHED)
> +                                ? PROCESS_CLOCK
>                                  : clock_id == CLOCK_THREAD_CPUTIME_ID
> -                                ? MAKE_THREAD_CPUCLOCK (0, CPUCLOCK_SCHED)
> +                                ? THREAD_CLOCK
>                                  : clock_id);
>
>      /* If the user wants notification via a thread we need to handle

OK. Use the new macros.