Re: nanosleep interrupted by ignored signals

[George Anzinger <george@mvista.com>]

Marcelo Tosatti wrote:
> On Wed, Nov 24, 2004 at 07:06:27PM -0800, Matt Mackall wrote:
> 
>>On Wed, Nov 24, 2004 at 06:45:05PM -0800, George Anzinger wrote:
>>
>>>Matt Mackall wrote:
>>>
>>>>Take the following trivial program:
>>>>
>>>>#include <unistd.h>
>>>>
>>>>int main(void)
>>>>{
>>>>	sleep(10);
>>>>	return 0;
>>>>}
>>>>
>>>>Run it in an xterm. Note that resizing the xterm has no effect on the
>>>>process. Now do the same with strace:
>>>>
>>>>brk(0x80495bc)                          = 0x80495bc
>>>>brk(0x804a000)                          = 0x804a000
>>>>rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
>>>>rt_sigaction(SIGCHLD, NULL, {SIG_DFL}, 8) = 0
>>>>rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
>>>>nanosleep({10, 0}, 0xbffff548)          = -1 EINTR (Interrupted system
>>>>call)
>>>>--- SIGWINCH (Window changed) ---
>>>>_exit(0)                                = ?
>>>>
>>>>In short, nanosleep is getting interrupted by signals that are
>>>>supposedly ignored when a process is being praced. This appears to be
>>>>a long-standing bug.
>>>>
>>>>It also appears to be a long-known bug. I found some old discussion of this
>>>>problem here but no sign of any resolution:
>>>>
>>>>http://www.ussg.iu.edu/hypermail/linux/kernel/0108.1/1448.html
>>>>
>>>>What's the current thinking on this?
>>>
>>>This should have been resolved with the 2.6 changes, in particular, the 
>>>restart code.  What kernel are you using?
>>
>>Indeed it is. Forgot I still had 2.4 on the box in question, didn't
>>notice the restart bit when comparing the 2.6 code against the thread
>>above. Mea culpa.
> 
> 
> George, 
> 
> Is it worth/necessary to fix this bug in v2.4 ?
> 
> Quoting yourself
> 
> "This is an issue for debugging also (same ptrace...). The fix is to fix
> nano_sleep to match the standard which says it should only return on a
> signal if the signal is delivered to the program (i.e. not on internal
> "do nothing" signals). Signal in the kernel returns 1 if it calls the
> task and 0 otherwise, thus nano sleep might be changed as follows: "
> 
Hmm,  wise fellow, that :)  We (MontaVista) have back ported this fix to our 
kernels as part of the HRT patch, and, in fact, it is in the latest (albeit 
somewhat out of date) HRT patch on sourceforge.  The main issue is that it 
requires changes in arch level code and so requires a cooperative effort (in 
that most folks only have one or two archs to check it out on).

My take on this is that this has been in the kernel since nanosleep() was put in 
and so, for a mature kernel, it is not really important to change it.  Now if 
you want to back port POSIX clocks and timers (i.e. clock_nanosleep()) I would 
argue that you should back port this change as part of that effort.

-- 
George Anzinger   george@mvista.com
High-res-timers:  http://sourceforge.net/projects/high-res-timers/