kern/181416: socket timeout rounding issue

[Vitja Makarov]

>Number:         181416
>Category:       kern
>Synopsis:       socket timeout rounding issue
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Tue Aug 20 04:40:00 UTC 2013
>Closed-Date:
>Last-Modified:
>Originator:     Vitja Makarov
>Release:        9.1-RELEASE
>Organization:
Rambler
>Environment:
FreeBSD dbg 9.1-RELEASE FreeBSD 9.1-RELEASE #5: Tue Aug 20 08:14:54 MSK 2013     root at dbg:/usr/obj/nfs/usr/src/sys/MYKERNEL  i386

>Description:
Recently I was playing with small socket timeouts. setsockopt(2)
SO_RCVTIMEO and found a problem with it: if timeout is small enough
read(2) may return before timeout is actually expired.

I was unable to reproduce this on linux box.

I found that kernel uses a timer with 1/HZ precision so it converts
time in microseconds to ticks that's ok linux does it as well. The
problem is in details: freebsd uses floor() approach while linux uses
ceil():

from FreeBSD's sys/kern/uipc_socket.c:
val = (u_long)(tv.tv_sec * hz) + tv.tv_usec / tick;
if (val == 0 && tv.tv_usec != 0)
     val = 1; /* at least one tick if tv > 0 */

from Linux's net/core/sock.c:
*timeo_p = tv.tv_sec*HZ + (tv.tv_usec+(1000000/HZ-1))/(1000000/HZ);

So, for instance, we have a freebsd system running with kern.hz set to
100 and set receive timeout to 25ms that is converted to 2 ticks which
is 20ms. In my test program read(2) returns with EAGAIN set in
0.019ms.

>How-To-Repeat:

>Fix:


>Release-Note:
>Audit-Trail:
>Unformatted: