[PATCH 3/3] sched, timer: Use cmpxchg to do updates in update_gt_cputime()

[Jason Low]

Note: The chance that the race which this patch addresses seems very
unlikely to occur, especially after the change in patch 2 which sets
the running field after calling this update_gt_cputimer().

However, I am including this patch if we want to be completely safe
from concurrent updates.

-----------------------------------------------------------------------------

Since we're now updating thread group cputimer values without a lock,
there is now a potential race that can occur in update_gt_cputime() where
the cputimers are concurrently being updated in account_group_*_time().

This can occur when the ->running field transitions from 1 -> 0 -> 1. If the
cputimer->running field is set while thread 1 runs run_posix_cpu_timers(),
but another thread, thread 2, turns off cputimer->running before thread 1
enters thread_group_cputimer(), and another thread, thread 3, enables it
after thread 1 checks !cputimer->running in thread_group_cputimer(), then
there is a possibility that update_gt_cputime() is updating the cputimers
while the cputimer is running.

This patch uses cmpxchg and retry logic to ensure that update_gt_cputime()
is making its updates atomically.

Signed-off-by: Jason Low <jason.low2@xxxxxx>
---
 kernel/time/posix-cpu-timers.c |   26 ++++++++++++++++++--------
 1 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 7e96082..130d717 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -196,16 +196,26 @@ static int cpu_clock_sample(const clockid_t which_clock, struct task_struct *p,
 	return 0;
 }
 
-static void update_gt_cputime(struct thread_group_cputimer *cputimer, struct task_cputime *sum)
+static inline void __update_gt_cputime(atomic64_t *cputime, u64 sum_cputime)
 {
-	if (sum->utime > atomic64_read(&cputimer->utime))
-		atomic64_set(&cputimer->utime, sum->utime);
-
-	if (sum->stime > atomic64_read(&cputimer->stime))
-		atomic64_set(&cputimer->stime, sum->stime);
+	u64 curr_cputime;
+	/*
+	 * Set cputime to sum_cputime if sum_cputime > cputime. Use cmpxchg
+	 * to avoid race conditions with concurrent updates to cputime.
+	 */
+retry:
+	curr_cputime = atomic64_read(cputime);
+	if (sum_cputime > curr_cputime) {
+		if (atomic64_cmpxchg(cputime, curr_cputime, sum_cputime) != curr_cputime)
+			goto retry;
+	}
+}
 
-	if (sum->sum_exec_runtime > atomic64_read(&cputimer->sum_exec_runtime))
-		atomic64_set(&cputimer->sum_exec_runtime, sum->sum_exec_runtime);
+static void update_gt_cputime(struct thread_group_cputimer *cputimer, struct task_cputime *sum)
+{
+	__update_gt_cputime(&cputimer->utime, sum->utime);
+	__update_gt_cputime(&cputimer->stime, sum->stime);
+	__update_gt_cputime(&cputimer->sum_exec_runtime, sum->sum_exec_runtime);
 }
 
 /* Sample thread_group_cputimer values in "cputimer", copy results to "times" */
-- 
1.7.2.5

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