Re: udevd on very large systems: again

[Kay Sievers <kay.sievers@vrfy.org>]

On Wed, 2007-05-02 at 17:26 -0400, George Beshers wrote:
> Kay Sievers wrote:
> > Looks fine so far, but I still don't see how reading a seq-proc-file
> > could miss entries. Are you expecting this to happen? Did you ever see
> > this for any of these files?

> The answer is slightly complex.  No I never saw it happen, but I am
> concerned about it.  To illustrate that, consider the following test
> program and the output log attached.  Basically, the /proc/stat file
> does seem to be changing dynamically and an open file does not
> present a stable snapshot.

Looking at the seq_file code, calling lseek() causes the buffer to be
repopulated:
  seq_lseek() -> traverse() -> m->op->show()
while show() is the method of the "stat" file that fills the buffer.

A simple read() should not do that, and just copy the content (up to
128kB) of the kernel seq_file file buffer to the user-process:
  seq_read() -> copy_to_user(buf, m->buf + m->from, n)

>  So the danger is that the procs_running
> might get split at a 4k boundary and the split changes by the time
> the next read occurs so procs_running is corrupted.

I'm pretty sure, that this doesn't happen, if you don't seek(). Here is
a test, that opens /proc/stat, reads a single byte (which fills the
buffer), forks a ton of processes and reads the rest of the buffer. You
see that the "processes" value did not increase:
  $ ./seq 
  processes 57425
  processes 57425
  processes 58203




#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>

int main(int argc, char *argv[])
{
        FILE* f;
        char buf[1024];
        int i;

        system("grep processes /proc/stat");

        f = fopen("/proc/stat", "r");
        fread(buf, 1, 1, f);

        for (i = 0; i < 777; i++)
                system("/bin/true");

        while (fgets(buf, sizeof(buf), f))
                if (strncmp(buf, "processes", 9) = 0)
                        printf("%s", buf);
        fclose(f);

        system("grep processes /proc/stat");

        return 0;
}


Thanks,
Kay


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