RISC-V: Reliably initialize t0 in _times()

Message ID 20210802164616.4099311-1-cmuellner@gcc.gnu.org
State New
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  • RISC-V: Reliably initialize t0 in _times()
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Commit Message

Christoph Muellner Aug. 2, 2021, 4:46 p.m.
From: Christoph Muellner <cmuellner@linux.com>


The current implementation does not reliably initialize t0 once.
Additionally the initialization requires two calls to _gettimeofday().
Let's sacrifice a byte to keep the initialization status
and reduce the maximum number of calls to _gettimeofday().

This has caused issues in an application that invokes clock().
The problematic situation is as follows:

1) The program calls clock() which calls _times().
2) _gettimeofday(&t0, 0) puts 0 in t0.tv_usec (because less than 1 us has
   elapsed since the beginning of time).
3) _gettimeofday(&t, 0) puts 1 in t.tv_usec (since now more than 1 us has
   elapsed since the beginning of time).
4) That call to clock() returns 1 (the value from step 3 minus the value in
   step 2).
5) The program does a second call to clock().
6) The code above still sees 0 in t0 so it tries to update t0 again and
   _gettimeofday(&t0, 0) puts 1 in t0.tv_usec.
7) The _gettimeofday(&t, 0) puts 1 in t.tv_usec (since less than 1us has
   elapsed since step 3).
8) clock() returns 0 (step 7 minus step 6) and indicates that time is
   moving backwards.

Signed-off-by: Christoph Muellner <cmuellner@gcc.gnu.org>

---
 libgloss/riscv/sys_times.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

-- 
2.31.1

Comments

Corinna Vinschen Aug. 4, 2021, 9:44 a.m. | #1
On Aug  2 18:46, Christoph Muellner wrote:
> From: Christoph Muellner <cmuellner@linux.com>

> 

> The current implementation does not reliably initialize t0 once.

> Additionally the initialization requires two calls to _gettimeofday().

> Let's sacrifice a byte to keep the initialization status

> and reduce the maximum number of calls to _gettimeofday().

> 

> This has caused issues in an application that invokes clock().

> The problematic situation is as follows:

> 

> 1) The program calls clock() which calls _times().

> 2) _gettimeofday(&t0, 0) puts 0 in t0.tv_usec (because less than 1 us has

>    elapsed since the beginning of time).

> 3) _gettimeofday(&t, 0) puts 1 in t.tv_usec (since now more than 1 us has

>    elapsed since the beginning of time).

> 4) That call to clock() returns 1 (the value from step 3 minus the value in

>    step 2).

> 5) The program does a second call to clock().

> 6) The code above still sees 0 in t0 so it tries to update t0 again and

>    _gettimeofday(&t0, 0) puts 1 in t0.tv_usec.

> 7) The _gettimeofday(&t, 0) puts 1 in t.tv_usec (since less than 1us has

>    elapsed since step 3).

> 8) clock() returns 0 (step 7 minus step 6) and indicates that time is

>    moving backwards.

> 

> Signed-off-by: Christoph Muellner <cmuellner@gcc.gnu.org>

> ---

>  libgloss/riscv/sys_times.c | 13 +++++++++----

>  1 file changed, 9 insertions(+), 4 deletions(-)


Pushed.


Thanks,
Corinna

Patch

diff --git a/libgloss/riscv/sys_times.c b/libgloss/riscv/sys_times.c
index fc8133ade..a029bcfb5 100644
--- a/libgloss/riscv/sys_times.c
+++ b/libgloss/riscv/sys_times.c
@@ -21,14 +21,19 @@  extern int _gettimeofday(struct timeval *, void *);
 clock_t
 _times(struct tms *buf)
 {
-  // when called for the first time, initialize t0
+  static char initialized;
   static struct timeval t0;
-  if (t0.tv_sec == 0 && t0.tv_usec == 0)
-    _gettimeofday (&t0, 0);
-
   struct timeval t;
+
   _gettimeofday (&t, 0);
 
+  // when called for the first time, initialize t0
+  if (!initialized) {
+    t0.tv_sec = t.tv_sec;
+    t0.tv_usec = t.tv_usec;
+    initialized = 1;
+  }
+
   long long utime = (t.tv_sec - t0.tv_sec) * 1000000 + (t.tv_usec - t0.tv_usec);
   buf->tms_utime = utime * CLOCKS_PER_SEC / 1000000;
   buf->tms_stime = buf->tms_cstime = buf->tms_cutime = 0;