Re: [Qemu-devel] [PATCH v8 7/7] block: Enable qemu_open/close to work with fd sets

[Kevin Wolf <kwolf@redhat.com>]

Am 10.08.2012 04:10, schrieb Corey Bryant:
> When qemu_open is passed a filename of the "/dev/fdset/nnn"
> format (where nnn is the fdset ID), an fd with matching access
> mode flags will be searched for within the specified monitor
> fd set.  If the fd is found, a dup of the fd will be returned
> from qemu_open.
> 
> Each fd set has a reference count.  The purpose of the reference
> count is to determine if an fd set contains file descriptors that
> have open dup() references that have not yet been closed.  It is
> incremented on qemu_open and decremented on qemu_close.  It is
> not until the refcount is zero that file desriptors in an fd set
> can be closed.  If an fd set has dup() references open, then we
> must keep the other fds in the fd set open in case a reopen
> of the file occurs that requires an fd with a different access
> mode.
> 
> Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>

> @@ -78,6 +79,69 @@ int qemu_madvise(void *addr, size_t len, int advice)
>  #endif
>  }
>  
> +/*
> + * Dups an fd and sets the flags
> + */
> +static int qemu_dup(int fd, int flags)

qemu_dup() is probably not a good name. We'll want to use it when we
need to get a wrapper around dup(). And I suspect that we will need it
for making bdrv_reopen() compatible with fdset refcounting.

> +{
> +    int ret;
> +    int serrno;
> +    int dup_flags;
> +    int setfl_flags;
> +
> +    if (flags & O_CLOEXEC) {
> +#ifdef F_DUPFD_CLOEXEC
> +        ret = fcntl(fd, F_DUPFD_CLOEXEC, 0);
> +#else
> +        ret = dup(fd);
> +        if (ret != -1) {
> +            qemu_set_cloexec(ret);
> +        }
> +#endif
> +    } else {
> +        ret = dup(fd);
> +    }

qemu_open() is supposed to add O_CLOEXEC by itself, so I think we should
execute the then branch unconditionally (or we would have to change the
qemu_dup() call below to add it - but the fact that O_CLOEXEC isn't even
necessarily defined doesn't help with that).

Kevin