Re: [PATCH -mm] swsusp: freeze user space processes first

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Sunday 05 February 2006 12:18, Pavel Machek wrote:
> On Ne 05-02-06 12:11:06, Rafael J. Wysocki wrote:
> > On Sunday 05 February 2006 11:50, Ingo Molnar wrote:
> > > 
> > > * Rafael J. Wysocki <rjw@sisk.pl> wrote:
> > > 
> > > > > The logic in that loop makes my brain burst.
> > > > > 
> > > > > What happens if a process does vfork();sleep(100000000)?
> > > > 
> > > > The freezing of processes will fail due to the timeout.
> > > > 
> > > > Without the if (!p->vfork_done) it would fail too, because the child 
> > > > would be frozen and the parent would wait for the vfork completion in 
> > > > the TASK_UNINTERRUPTIBLE state (ie. unfreezeable).  But in that case 
> > > > we have a race between the "freezer" and the child process (ie. if the 
> > > > child gets frozen before it completes the vfork completion, the paret 
> > > > will be unfreezeable) which sometimes leads to a failure when it 
> > > > should not.  [We have a test case showing this.]
> > > 
> > > then i'd suggest to change the vfork implementation to make this code 
> > > freezable.
> > 
> > I think you are right, but I don't know how to do this.
> > 
> > > Nothing that userspace does should cause freezing to fail.   If it does,
> > > we've designed things incorrectly on the kernel side. 
> > 
> > I tend to agree.
> > 
> > Generally, the problem is due to the use of completions where userland
> > processes are waited for.  The two places I know of are the vfork
> > implementation and the usermode helper code.
> 
> Can you produce userland testcase? If we have uninterruptible process for
> days... that's a bug in kernel, suspend or not.

Sure, no problem.  [Pretty scary, no?]

The test code:

#include <sys/types.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
	vfork();
	sleep(300);

	return 0;
}

The result:

rafael@albercik:~/programming/c> ./vfork_test &
[1] 12288
rafael@albercik:~/programming/c> ps l
F   UID   PID  PPID PRI  NI    VSZ   RSS WCHAN  STAT TTY        TIME COMMAND
0   500  6937  6931  17   0  10048  2368 read_c Ss+  pts/1      0:00 /bin/bash
0   500 12139 12133  15   0  10052  2380 wait   Ss   pts/2      0:00 /bin/bash
0   500 12288 12139  15   0   2420   304 fork   D    pts/2      0:00 ./vfork_tes
1   500 12291 12288  18   0   2420   304 hrtime S    pts/2      0:00 ./vfork_tes
0   500 12372 12139  15   0   3596   820 -      R+   pts/2      0:00 ps l
rafael@albercik:~/programming/c> kill 12288
rafael@albercik:~/programming/c> ps l
F   UID   PID  PPID PRI  NI    VSZ   RSS WCHAN  STAT TTY        TIME COMMAND
0   500  6937  6931  17   0  10048  2368 read_c Ss+  pts/1      0:00 /bin/bash
0   500 12139 12133  15   0  10052  2380 wait   Ss   pts/2      0:00 /bin/bash
0   500 12288 12139  15   0   2420   304 fork   D    pts/2      0:00 ./vfork_tes
1   500 12291 12288  18   0   2420   304 hrtime S    pts/2      0:00 ./vfork_tes
0   500 12380 12139  17   0   3600   820 -      R+   pts/2      0:00 ps l
rafael@albercik:~/programming/c> kill -9 12288
rafael@albercik:~/programming/c> ps l
F   UID   PID  PPID PRI  NI    VSZ   RSS WCHAN  STAT TTY        TIME COMMAND
0   500  6937  6931  17   0  10048  2368 read_c Ss+  pts/1      0:00 /bin/bash
0   500 12139 12133  15   0  10052  2380 wait   Ss   pts/2      0:00 /bin/bash
0   500 12288 12139  15   0   2420   304 fork   D    pts/2      0:00 ./vfork_tes
1   500 12291 12288  18   0   2420   304 hrtime S    pts/2      0:00 ./vfork_tes
0   500 12387 12139  16   0   3596   816 -      R+   pts/2      0:00 ps l
rafael@albercik:~/programming/c>

Greetings,
Rafael