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authorThomas Gleixner <tglx@linutronix.de>2006-11-05 09:02:46 +0100
committerAdrian Bunk <bunk@stusta.de>2006-11-05 09:02:46 +0100
commitfe8187b80c7b4fee6fd5dfb0d68891230ae59dcb (patch)
tree20d3016501a4486526cedb4a6cd5095d1fb42a51 /kernel
parentd1ce361a6c0115e2ef500e9e7dfe77763d287fcd (diff)
downloadlinux-linaro-stable-fe8187b80c7b4fee6fd5dfb0d68891230ae59dcb.tar.gz
posix-cpu-timers: prevent signal delivery starvation
The integer divisions in the timer accounting code can round the result down to 0. Adding 0 is without effect and the signal delivery stops. Clamp the division result to minimum 1 to avoid this. Problem was reported by Seongbae Park <spark@google.com>, who provided also an inital patch. Roland sayeth: I have had some more time to think about the problem, and to reproduce it using Toyo's test case. For the record, if my understanding of the problem is correct, this happens only in one very particular case. First, the expiry time has to be so soon that in cputime_t units (usually 1s/HZ ticks) it's < nthreads so the division yields zero. Second, it only affects each thread that is so new that its CPU time accumulation is zero so now+0 is still zero and ->it_*_expires winds up staying zero. For the VIRT and PROF clocks when cputime_t is tick granularity (or the SCHED clock on configurations where sched_clock's value only advances on clock ticks), this is not hard to arrange with new threads starting up and blocking before they accumulate a whole tick of CPU time. That's what happens in Toyo's test case. Note that in general it is fine for that division to round down to zero, and set each thread's expiry time to its "now" time. The problem only arises with thread's whose "now" value is still zero, so that now+0 winds up 0 and is interpreted as "not set" instead of ">= now". So it would be a sufficient and more precise fix to just use max(ticks, 1) inside the loop when setting each it_*_expires value. But, it does no harm to round the division up to one and always advance every thread's expiry time. If the thread didn't already fire timers for the expiry time of "now", there is no expectation that it will do so before the next tick anyway. So I followed Thomas's patch in lifting the max out of the loops. This patch also covers the reload cases, which are harder to write a test for (and I didn't try). I've tested it with Toyo's case and it fixes that. [toyoa@mvista.com: fix: min_t -> max_t] Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Roland McGrath <roland@redhat.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Adrian Bunk <bunk@stusta.de>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/posix-cpu-timers.c27
1 files changed, 21 insertions, 6 deletions
diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 4882bf1e094a..11f36820c791 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -88,6 +88,19 @@ static inline union cpu_time_count cpu_time_sub(const clockid_t which_clock,
}
/*
+ * Divide and limit the result to res >= 1
+ *
+ * This is necessary to prevent signal delivery starvation, when the result of
+ * the division would be rounded down to 0.
+ */
+static inline cputime_t cputime_div_non_zero(cputime_t time, unsigned long div)
+{
+ cputime_t res = cputime_div(time, div);
+
+ return max_t(cputime_t, res, 1);
+}
+
+/*
* Update expiry time from increment, and increase overrun count,
* given the current clock sample.
*/
@@ -483,8 +496,8 @@ static void process_timer_rebalance(struct task_struct *p,
BUG();
break;
case CPUCLOCK_PROF:
- left = cputime_div(cputime_sub(expires.cpu, val.cpu),
- nthreads);
+ left = cputime_div_non_zero(cputime_sub(expires.cpu, val.cpu),
+ nthreads);
do {
if (likely(!(t->flags & PF_EXITING))) {
ticks = cputime_add(prof_ticks(t), left);
@@ -498,8 +511,8 @@ static void process_timer_rebalance(struct task_struct *p,
} while (t != p);
break;
case CPUCLOCK_VIRT:
- left = cputime_div(cputime_sub(expires.cpu, val.cpu),
- nthreads);
+ left = cputime_div_non_zero(cputime_sub(expires.cpu, val.cpu),
+ nthreads);
do {
if (likely(!(t->flags & PF_EXITING))) {
ticks = cputime_add(virt_ticks(t), left);
@@ -515,6 +528,7 @@ static void process_timer_rebalance(struct task_struct *p,
case CPUCLOCK_SCHED:
nsleft = expires.sched - val.sched;
do_div(nsleft, nthreads);
+ nsleft = max_t(unsigned long long, nsleft, 1);
do {
if (likely(!(t->flags & PF_EXITING))) {
ns = t->sched_time + nsleft;
@@ -1162,12 +1176,13 @@ static void check_process_timers(struct task_struct *tsk,
prof_left = cputime_sub(prof_expires, utime);
prof_left = cputime_sub(prof_left, stime);
- prof_left = cputime_div(prof_left, nthreads);
+ prof_left = cputime_div_non_zero(prof_left, nthreads);
virt_left = cputime_sub(virt_expires, utime);
- virt_left = cputime_div(virt_left, nthreads);
+ virt_left = cputime_div_non_zero(virt_left, nthreads);
if (sched_expires) {
sched_left = sched_expires - sched_time;
do_div(sched_left, nthreads);
+ sched_left = max_t(unsigned long long, sched_left, 1);
} else {
sched_left = 0;
}