[PATCH] sched, time: fix build error with 64 bit cputime_t on 32 bit systems

[Rik van Riel]

On Tue, 30 Sep 2014 13:56:37 +0200
Arnd Bergmann <arnd@xxxxxxxx> wrote:

> A recent change to update the stime/utime members of task_struct
> using atomic cmpxchg broke configurations on 32-bit machines with
> CONFIG_VIRT_CPU_ACCOUNTING_GEN set, because that uses 64-bit
> nanoseconds, leading to a link-time error:
> 
> kernel/built-in.o: In function `cputime_adjust':
> :(.text+0x25234): undefined reference to `__bad_cmpxchg'

Arnd, this should fix your problem, while still ensuring that
the cpu time counters only ever go forward.

I do not have cross compiling toolchains set up here, but I assume
this fixes your bug.

Ingo & Peter, if this patch fixes the bug for Arnd, could you please
merge it into -tip?

Linus, the changeset causing the problem is only in -tip right now,
and this patch will not apply to your tree.

---8<---

Subject: sched,time: fix build error with 64 bit cputime_t on 32 bit systems

On 32 bit systems cmpxchg cannot handle 64 bit values, and
cmpxchg64 needs to be used when full dynticks CPU accounting
is enabled, since that turns cputime_t into a u64.

With jiffies based CPU accounting, cputime_t is an unsigned
long. On 64 bit systems, cputime_t is always the size of a
long.

Luckily the compiler can figure out whether we need to call
cmpxchg or cmpxchg64.

Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
---
 kernel/sched/cputime.c | 35 +++++++++++++++++++++++++----------
 1 file changed, 25 insertions(+), 10 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 64492df..db239c9 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -555,6 +555,29 @@ static cputime_t scale_stime(u64 stime, u64 rtime, u64 total)
 }
 
 /*
+ * Atomically advance counter to the new value. Interrupts, vcpu
+ * scheduling, and scaling inaccuracies can cause cputime_advance
+ * to be occasionally called with a new value smaller than counter.
+ * Let's enforce atomicity.
+ *
+ * Normally a caller will only go through this loop once, or not
+ * at all in case a previous caller updated counter the same jiffy.
+ */
+static void cputime_advance(cputime_t *counter, cputime_t new)
+{
+	cputime_t old;
+
+	while (new > (old = ACCESS_ONCE(*counter))) {
+		/* The compiler will optimize away this branch.  */
+		if (sizeof(cputime_t) == sizeof(long))
+			cmpxchg(counter, old, new);
+		else
+			/* 64 bit cputime_t on a 32 bit system... */
+			cmpxchg64(counter, old, new);
+	}
+}
+
+/*
  * Adjust tick based cputime random precision against scheduler
  * runtime accounting.
  */
@@ -599,16 +622,8 @@ static void cputime_adjust(struct task_cputime *curr,
 		utime = rtime - stime;
 	}
 
-	/*
-	 * If the tick based count grows faster than the scheduler one,
-	 * the result of the scaling may go backward.
-	 * Let's enforce monotonicity.
-	 * Atomic exchange protects against concurrent cputime_adjust().
-	 */
-	while (stime > (rtime = ACCESS_ONCE(prev->stime)))
-		cmpxchg(&prev->stime, rtime, stime);
-	while (utime > (rtime = ACCESS_ONCE(prev->utime)))
-		cmpxchg(&prev->utime, rtime, utime);
+	cputime_advance(&prev->stime, stime);
+	cputime_advance(&prev->utime, utime);
 
 out:
 	*ut = prev->utime;
--
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