[PATCH RFC 1/3] nptl: Introduce futex_lock_pi2()

[Kurt Kanzenbach <kurt@linutronix.de>]

This variant of futex_lock() has support for selectable clocks and priority
inheritance. The underlying FUTEX_LOCK_PI2 operation has been recently
introduced into the Linux kernel.

It can be used for implementing pthread_mutex_clocklock(MONOTONIC)/PI.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
---
 sysdeps/nptl/futex-internal.h     | 77 +++++++++++++++++++++++++++++++
 sysdeps/nptl/lowlevellock-futex.h |  1 +
 2 files changed, 78 insertions(+)

diff --git a/sysdeps/nptl/futex-internal.h b/sysdeps/nptl/futex-internal.h
index 969ab2bf4bf8..60bdae84405c 100644
--- a/sysdeps/nptl/futex-internal.h
+++ b/sysdeps/nptl/futex-internal.h
@@ -295,6 +295,83 @@ futex_lock_pi64 (int *futex_word, const struct __timespec64 *abstime,
     }
 }
 
+/* The operation checks the value of the futex, if the value is 0, then
+   it is atomically set to the caller's thread ID.  If the futex value is
+   nonzero, it is atomically sets the FUTEX_WAITERS bit, which signals wrt
+   other futex owner that it cannot unlock the futex in user space by
+   atomically by setting its value to 0.
+
+   If more than one wait operations is issued, the enqueueing of the waiters
+   are done in descending priority order.
+
+   The ABSTIME arguments provides an absolute timeout (measured against the
+   CLOCK_REALTIME or CLOCK_MONOTONIC clock).  If TIMEOUT is NULL, the operation
+   will block indefinitely.
+
+   Returns:
+
+     - 0 if woken by a PI unlock operation or spuriously.
+     - EAGAIN if the futex owner thread ID is about to exit, but has not yet
+       handled the state cleanup.
+     - EDEADLK if the futex is already locked by the caller.
+     - ESRCH if the thread ID int he futex does not exist.
+     - EINVAL is the state is corrupted or if there is a waiter on the
+       futex or if the clockid is invalid.
+     - ETIMEDOUT if the ABSTIME expires.
+     - ENOSYS if FUTEX_LOCK_PI2 is not available.
+*/
+static __always_inline int
+futex_lock_pi2_64 (int *futex_word, clockid_t clockid,
+		   const struct __timespec64 *abstime,
+		   int private)
+{
+  unsigned int clockbit;
+
+  if (! lll_futex_supported_clockid (clockid))
+    return EINVAL;
+
+  clockbit = (clockid == CLOCK_REALTIME) ? FUTEX_CLOCK_REALTIME : 0;
+  int op = __lll_private_flag (FUTEX_LOCK_PI2 | clockbit, private);
+
+  int err = INTERNAL_SYSCALL_CALL (futex_time64, futex_word, op,
+				   0, abstime);
+#ifndef __ASSUME_TIME64_SYSCALLS
+  if (err == -ENOSYS)
+    {
+      if (abstime != NULL && ! in_time_t_range (abstime->tv_sec))
+        return EOVERFLOW;
+
+      struct timespec ts32;
+      if (abstime != NULL)
+        ts32 = valid_timespec64_to_timespec (*abstime);
+
+      err = INTERNAL_SYSCALL_CALL (futex, futex_word, op, 0,
+                                   abstime != NULL ? &ts32 : NULL);
+    }
+#endif
+  switch (err)
+    {
+    case 0:
+    case -EAGAIN:
+    case -EINTR:
+    case -ETIMEDOUT:
+    case -ESRCH:
+    case -EDEADLK:
+    case -ENOSYS: /* Futex lock pi 2 is not available on this kernel. */
+    case -EINVAL: /* This indicates either state corruption or that the kernel
+		     found a waiter on futex address which is waiting via
+		     FUTEX_WAIT or FUTEX_WAIT_BITSET.  This is reported on
+		     some futex_lock_pi usage (pthread_mutex_timedlock for
+		     instance).  */
+      return -err;
+
+    case -EFAULT: /* Must have been caused by a glibc or application bug.  */
+    /* No other errors are documented at this time.  */
+    default:
+      futex_fatal_error ();
+    }
+}
+
 /* Wakes the top priority waiter that called a futex_lock_pi operation on
    the futex.
 
diff --git a/sysdeps/nptl/lowlevellock-futex.h b/sysdeps/nptl/lowlevellock-futex.h
index 66ebfe50f4c1..abda179e0de2 100644
--- a/sysdeps/nptl/lowlevellock-futex.h
+++ b/sysdeps/nptl/lowlevellock-futex.h
@@ -38,6 +38,7 @@
 #define FUTEX_WAKE_BITSET	10
 #define FUTEX_WAIT_REQUEUE_PI   11
 #define FUTEX_CMP_REQUEUE_PI    12
+#define FUTEX_LOCK_PI2		13
 #define FUTEX_PRIVATE_FLAG	128
 #define FUTEX_CLOCK_REALTIME	256
 
-- 
2.30.2