2018-07-29 12:40:11

by Kenneth Johansson

Subject: F_SETLK blocks on nfsv4

I was trying to change over to using nfsv4 from nfsv3 and hit a problem
with VLC. after some strace digging I came up with a test program that
has the same effect and works on nfs3 but hangs on nfsv4.

Even more interesting when run on a nfsv4 mounted home directory it is
no longer possible to open new connections in google-chrome so the error
is visible not only for the test program but probably all program that
does locking after this program is started on the same mount. or
something I have not investigated what really happens.

anyway the program is below and it hangs on last the line "ret =
"fcntl(fd, F_SETLK, &fl);" if run on a nfsv4 mount.

server; debain 9.5 with kernel version 4.9 or 4.17.0-0.bpo.1-amd64.

client: ubuntu 18.04. kernel 4.18.0-041800rc6-generic

--------------

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/file.h>
#include <string.h>

char *lfile="lock";

int main(int argc, char *argv[])
{
        int fd;
        int ret;
        struct flock fl;

        fd = openat(AT_FDCWD, lfile, O_RDWR|O_CREAT|O_EXCL|O_CLOEXEC,
0600);
        if (fd <0) {perror("");exit(1);}

        ret = fcntl(fd, F_SETFD, FD_CLOEXEC);
        if (ret <0) {perror("");exit(1);}

        ret = flock(fd, LOCK_EX|LOCK_NB);
        if (ret <0) {perror("");exit(1);}

        memset(&fl,0,sizeof(fl));
        fl.l_type = F_WRLCK;
        fl.l_whence=SEEK_SET;
        ret = fcntl(fd, F_SETLK, &fl);
        if (ret <0) {perror("");exit(1);}

        printf("done\n");
}

2018-07-29 13:51:58

by Jeffrey Layton

Subject: Re: F_SETLK blocks on nfsv4

On Sun, 2018-07-29 at 12:48 +0200, Kenneth Johansson wrote:
> I was trying to change over to using nfsv4 from nfsv3 and hit a problem
> with VLC. after some strace digging I came up with a test program that
> has the same effect and works on nfs3 but hangs on nfsv4.
>
> Even more interesting when run on a nfsv4 mounted home directory it is
> no longer possible to open new connections in google-chrome so the error
> is visible not only for the test program but probably all program that
> does locking after this program is started on the same mount. or
> something I have not investigated what really happens.
>
> anyway the program is below and it hangs on last the line "ret =
> "fcntl(fd, F_SETLK, &fl);" if run on a nfsv4 mount.
>
> server; debain 9.5 with kernel version 4.9 or 4.17.0-0.bpo.1-amd64.
>
> client: ubuntu 18.04. kernel 4.18.0-041800rc6-generic
>
> --------------
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <errno.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <sys/file.h>
> #include <string.h>
>
> char *lfile="lock";
>
> int main(int argc, char *argv[])
> {
> int fd;
> int ret;
> struct flock fl;
>
> fd = openat(AT_FDCWD, lfile, O_RDWR|O_CREAT|O_EXCL|O_CLOEXEC,
> 0600);
> if (fd <0) {perror("");exit(1);}
>
> ret = fcntl(fd, F_SETFD, FD_CLOEXEC);
> if (ret <0) {perror("");exit(1);}
>
> ret = flock(fd, LOCK_EX|LOCK_NB);
> if (ret <0) {perror("");exit(1);}
>
> memset(&fl,0,sizeof(fl));
> fl.l_type = F_WRLCK;
> fl.l_whence=SEEK_SET;
> ret = fcntl(fd, F_SETLK, &fl);
> if (ret <0) {perror("");exit(1);}
>
> printf("done\n");
> }
>
>

Thanks for the report. This looks like a client-side regression in
v4.18. On a hunch, I just backed out this patch and it seems to fix the
problem:

commit a3cf9bca2ace0351c4a4c17fbca4d652c323d5e5
Author: Benjamin Coddington <[email protected]>
Date: Thu May 3 07:12:57 2018 -0400

NFSv4: Don't add a new lock on an interrupted wait for LOCK

Ben, can you take a look?
--
Jeff Layton <[email protected]>