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SubjectRe: [PATCH] time/fs - file's time race with vgettimeofday
On Fri, Jul 02, 2010 at 06:14:22PM +0200, Oleg Nesterov wrote:
> In no way I can review this patch, but I am curious and have the questions.
> Also, I think it makes sense to cc the fs/ developers, I've added Al.

thanks

>
> On 07/02, Jiri Olsa wrote:
> >
> > there's a race among calling gettimeofday(2) and a file's time
> > updates. Following test program expose the race.
> >
> > run it in the while loop
> > while [ 1 ]; do ./test1 || break; done
> >
> > --- SNIP ---
> > #include <stdio.h>
> > #include <stdlib.h>
> > #include <fcntl.h>
> >
> > int main (void)
> > {
> > struct stat st;
> > struct timeval tv;
> >
> > unlink("./file");
> >
> > gettimeofday(&tv, NULL);
> >
> > if (-1 == creat("./file", O_RDWR)) {
> > perror("creat");
> > return -1;
> > }
> >
> > if (stat("./file", &st) != 0) {
> > perror("stat");
> > return -1;
> > }
> >
> > printf("USER gtod: %ld\n", (long)tv.tv_sec);
> > printf("USER file: %ld.%09u\n",
> > (long) st.st_mtime,
> > (unsigned) st.st_mtim.tv_nsec);
> >
> > return tv.tv_sec <= st.st_mtime ? 0 : -1;
> > }
>
> Interesting. To the point, I actually compiled this code and yes,
> it triggers the problem on ext3 ;)
>
> > The following patch will prevent the race by adding the
> > CURRENT_TIME_SEC_REAL macro, which will return seconds from
> > the getnstimeofday call, ensuring it's computed on current tick.
> > It fixes the 'creat' case for ext4.
>
> What about other filesystems? Perhaps it makes sense to change
> CURRENT_TIME_SEC instead of adding CURRENT_TIME_SEC_REAL?
>
> Once again, I am asking. It is not that I suggest to change your patch.

well, the patch is more or less to prove the problem exists and
it could be fixed :) also I'm not sure that all the places using
CURRENT_TIME_SEC suffer the same issue..

I'm currenty looking to the code and trying to come up with better
solution.. any ideas are welcome :)


jirka


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