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Re: [Qemu-block] [PATCH] block/nfs: add support for nfs_umount

From: Peter Lieven
Subject: Re: [Qemu-block] [PATCH] block/nfs: add support for nfs_umount
Date: Thu, 5 Sep 2019 12:32:12 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

Am 05.09.19 um 12:28 schrieb ronnie sahlberg:
On Thu, Sep 5, 2019 at 8:16 PM Peter Lieven <address@hidden> wrote:
Am 05.09.19 um 12:05 schrieb ronnie sahlberg:
On Thu, Sep 5, 2019 at 7:43 PM Peter Lieven <address@hidden> wrote:
Am 04.09.19 um 11:34 schrieb Kevin Wolf:
Am 03.09.2019 um 21:52 hat Peter Lieven geschrieben:
Am 03.09.2019 um 16:56 schrieb Kevin Wolf <address@hidden>:

Am 03.09.2019 um 15:44 hat Peter Lieven geschrieben:
libnfs recently added support for unmounting. Add support
in Qemu too.

Signed-off-by: Peter Lieven <address@hidden>
Looks trivial enough to review even for me. :-)

Thanks, applied to the block branch.

I am not sure what the reason is, but with this patch I sometimes run
into nfs_process_read being called for a cdrom mounted from nfs after
I ejected it (and the whole nfs client context is already destroyed).
Does this mean that nfs_umount() gets some response, but we don't
properly wait for it? Or is some older request still in flight?
nfs_umount itself is a sync call and should only terminate when

the call is done. But there is an independent I/O handler in that

function polling on the fd. (wait_for_nfs_reply in libnfs-sync.c).

This is why I thought the right solution is to stop the Qemu I/O handler

before calling nfs_close and nfs_umount. nfs_close also uses this

sync I/O handler, but for some reason it seems not to make trouble.

The other solution would be to use the async versions of close and umount,

but that would make the code in Qemu more complex.

NFS umount is pretty messy so I think you should continue using the
sync version.
In NFSv3 (there is no mount protocol in v4)  the Mount call (fetch the
root filehandle)
and the Umount calls (tell server we should no longer show up in
showexports -a output)
are not part of the NFS protocol but a different service running on a
separate port.

This does not map well to libnfs since it is centered around a "struct

To use nfs_umount() from QEMU I would suggest :
1, make sure all commands in flight have finished, because you will
soon disconnect from the NFS server and will never receive any
in-flight responses.
2, unregister the nfs->fh filedescriptor from your eventsystem.
Because the fd is about to be closed so there is great chance it will
be recycled for a completely different purpose if you open any other
files from qemu.

3, call nfs_umount()   Internally this will close the socket to the
NFS server, then go through thr process to open a new socket to the
portmapper to discover the mount server, then close that socket and
reconnect a new socket again to the mount server and perform the UMNT

What we currently do in Qemu is:

1) bdrv_drain

2) bdrv_close which in the end calls nfs_client_close from block/nfs.c.

    There we call:

    2a) nfs_close(client->fh)

    2b) aio_set_fd_handler(NULL)

    2c) nfs_destroy_context(client->context);

My first patch added a nfs_umount between 2a) and 2b) so that we have

    2a) nfs_close(client->fh)

    2b) nfs_umount(client->context)

    2c) aio_set_fd_handler(NULL)

    2d) nfs_destroy_context(client->context);

This leads to triggering to assertion for an uninitialized client->mutex which 
is called from an invocation

of nfs_process_read after nfs_destroy_context was called.

If I change the order as following I see no more assertions:

    2a) aio_set_fd_handler(NULL)

    2b) nfs_close(client->fh)

    2c) nfs_umount(client->context)

    2d) nfs_destroy_context(client->context);
That makes sense and looks correct to me.

I am unsure if we should acquire the client->mutex before we fiddle with

the aio_handler because in theory there could be something happen to the

nfs_fd in the background.



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