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Re: [PATCH 1/2] file-posix: Use OFD lock only if the filesystem supports
From: |
Daniel P . Berrangé |
Subject: |
Re: [PATCH 1/2] file-posix: Use OFD lock only if the filesystem supports the lock |
Date: |
Wed, 18 Nov 2020 15:16:53 +0000 |
User-agent: |
Mutt/1.14.6 (2020-07-11) |
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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