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Re: [libvirt] [PATCHv3] util: make it more robust to calculate timeout value



On Mon, May 25, 2015 at 02:22:42PM +0800, Wang Yufei wrote:
From: Zhang Bo <oscar zhangbo huawei com>

When we change system clock to years ago, a certain CPU may use up 100% cputime.
The reason is that in function virEventPollCalculateTimeout(), we assign the
unsigned long long result to an INT variable,
       *timeout = then - now; // timeout is INT, and then/now are long long
       if (*timeout < 0)
           *timeout = 0;
there's a chance that variable @then minus variable @now may be a very large number
that overflows INT value expression, then *timeout will be negative and be assigned to 0.
Next the 'poll' in function virEventPollRunOnce() will get into an 'endless' while loop there.
thus, the cpu that virEventPollRunOnce() thread runs on will go up to 100%.

Although as we discussed before in https://www.redhat.com/archives/libvir-list/2015-May/msg00400.html
it should be prohibited to set-time while other applications are running, but it does
seems to have no harm to make the codes more robust.

Signed-off-by: Wang Yufei <james wangyufei huawei com>
Signed-off-by: Zhang Bo <oscar zhangbo huawei com>
---
src/util/vireventpoll.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/util/vireventpoll.c b/src/util/vireventpoll.c
index ffda206..4e4f407 100644
--- a/src/util/vireventpoll.c
+++ b/src/util/vireventpoll.c
@@ -357,9 +357,10 @@ static int virEventPollCalculateTimeout(int *timeout)
            return -1;

        EVENT_DEBUG("Schedule timeout then=%llu now=%llu", then, now);
-        *timeout = then - now;
-        if (*timeout < 0)
+        if (then <= now)
            *timeout = 0;
+        else
+            *timeout = ((then-now) > INT_MAX) ? INT_MAX : (then-now);

This will work, although the following would be nicer to read since
there were such problems with the calculation in earlier versions:

 if (then <= now) {
     *timeout = 0;
 } else {
     long long tmp = then - now;
     if (tmp > INT_MAX)
         *timeout = INT_MAX
     else
         *timeout = tmp;
 }

But that, of course, has the same meaning as your version, which is
fine.  ACK to that, I'll push it in a while.

    } else {
        *timeout = -1;
    }
--
1.7.12.4


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