Re: [PATCH 1/2] file-posix: Use OFD lock only if the filesystem supports the lock

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Nov 05, 2020 at 11:01:01PM -0500, Masayoshi Mizuma wrote:
> From: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
> 
> locking=auto doesn't work if the filesystem doesn't support OFD lock.
> In that situation, following error happens:
> 
>   qemu-system-x86_64: -blockdev driver=qcow2,node-name=disk,file.driver=file,file.filename=/mnt/guest.qcow2,file.locking=auto: Failed to lock byte 100
> 
> qemu_probe_lock_ops() judges whether qemu can use OFD lock
> or not with doing fcntl(F_OFD_GETLK) to /dev/null. So the
> error happens if /dev/null supports OFD lock, but the filesystem
> doesn't support the lock.
> 
> Lock the actual file, not /dev/null, using F_OFD_SETLK and if that
> fails, then fallback to F_SETLK.
> 
> Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
> ---
>  block/file-posix.c   |  56 ++++++++--------
>  include/qemu/osdep.h |   2 +-
>  util/osdep.c         | 149 ++++++++++++++++++++++++++++---------------
>  3 files changed, 125 insertions(+), 82 deletions(-)


> diff --git a/util/osdep.c b/util/osdep.c
> index 66d01b9160..454e8ef9f4 100644
> --- a/util/osdep.c
> +++ b/util/osdep.c
> @@ -117,9 +117,6 @@ int qemu_mprotect_none(void *addr, size_t size)
>  
>  #ifndef _WIN32
>  
> -static int fcntl_op_setlk = -1;
> -static int fcntl_op_getlk = -1;
> -
>  /*
>   * Dups an fd and sets the flags
>   */
> @@ -187,68 +184,87 @@ static int qemu_parse_fdset(const char *param)
>      return qemu_parse_fd(param);
>  }
>  
> -static void qemu_probe_lock_ops(void)
> +bool qemu_has_ofd_lock(int orig_fd)
>  {
> -    if (fcntl_op_setlk == -1) {
>  #ifdef F_OFD_SETLK
> -        int fd;
> -        int ret;
> -        struct flock fl = {
> -            .l_whence = SEEK_SET,
> -            .l_start  = 0,
> -            .l_len    = 0,
> -            .l_type   = F_WRLCK,
> -        };
> -
> -        fd = open("/dev/null", O_RDWR);
> -        if (fd < 0) {
> +    int fd;
> +    int ret;
> +    struct flock fl = {
> +        .l_whence = SEEK_SET,
> +        .l_start  = 0,
> +        .l_len    = 0,
> +        .l_type   = F_RDLCK,
> +    };
> +
> +    fd = qemu_dup(orig_fd);

Consider that we're *not* using  OFD locks, and QEMU already
has 'foo.qcow2' open for an existing disk backend, and it is
locked.

Now someone tries to hot-add 'foo.qcow2' for a second disk
by mistake.  Doing this qemu_dup + qemu_close will cause
the existing locks to be removed AFAICT.

> +    if (fd >= 0) {
> +        ret = fcntl_setfl(fd, O_RDONLY);
> +        if (ret) {
>              fprintf(stderr,
> -                    "Failed to open /dev/null for OFD lock probing: %s\n",
> -                    strerror(errno));
> -            fcntl_op_setlk = F_SETLK;
> -            fcntl_op_getlk = F_GETLK;
> -            return;
> -        }
> -        ret = fcntl(fd, F_OFD_GETLK, &fl);
> -        close(fd);
> -        if (!ret) {
> -            fcntl_op_setlk = F_OFD_SETLK;
> -            fcntl_op_getlk = F_OFD_GETLK;
> -        } else {
> -            fcntl_op_setlk = F_SETLK;
> -            fcntl_op_getlk = F_GETLK;
> +                    "Failed to fcntl for OFD lock probing.\n");
> +            qemu_close(fd);
> +            return false;
>          }
> +    }
> +
> +    ret = fcntl(fd, F_OFD_GETLK, &fl);
> +    qemu_close(fd);
> +
> +    if (ret == 0) {
> +        return true;
> +    } else {
> +        return false;
> +    }
>  #else
> -        fcntl_op_setlk = F_SETLK;
> -        fcntl_op_getlk = F_GETLK;
> +    return false;
>  #endif
> -    }
>  }
>  
> -bool qemu_has_ofd_lock(void)
> -{
> -    qemu_probe_lock_ops();
>  #ifdef F_OFD_SETLK
> -    return fcntl_op_setlk == F_OFD_SETLK;
> +static int _qemu_lock_fcntl(int fd, struct flock *fl)
> +{
> +    int ret;
> +    bool ofd_lock = true;
> +
> +    do {
> +        if (ofd_lock) {
> +            ret = fcntl(fd, F_OFD_SETLK, fl);
> +            if ((ret == -1) && (errno == EINVAL)) {
> +                ofd_lock = false;
> +            }
> +        }
> +
> +        if (!ofd_lock) {
> +            /* Fallback to POSIX lock */
> +            ret = fcntl(fd, F_SETLK, fl);
> +        }
> +    } while (ret == -1 && errno == EINTR);

THis loop is confusing to read. I'd suggest creating a
wrapper

  qemu_fcntl()

that does the while (ret == -1 && errno == EINTR) loop,
so that this locking code can be clearer without the
loop.

> +
> +    return ret == -1 ? -errno : 0;
> +}
>  #else
> -    return false;
> -#endif
> +static int _qemu_lock_fcntl(int fd, struct flock *fl)
> +{
> +    int ret;
> +
> +    do {
> +        ret = fcntl(fd, F_SETLK, fl);
> +    } while (ret == -1 && errno == EINTR);
> +
> +    return ret == -1 ? -errno : 0;
>  }
> +#endif


Regards,
Daniel
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