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From: | Corey Bryant |
Subject: | Re: [Qemu-devel] [PATCH v4 0/7] file descriptor passing using pass-fd |
Date: | Tue, 03 Jul 2012 14:21:17 -0400 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120615 Thunderbird/13.0.1 |
On 07/03/2012 02:00 PM, Eric Blake wrote:
On 07/03/2012 11:46 AM, Corey Bryant wrote:Yes, I think adding a +1 to the refcount for the monitor makes sense. I'm a bit unsure how to increment the refcount when a monitor reconnects though. Maybe it is as simple as adding a +1 to each fd's refcount when the next QMP monitor connects.Or maybe delay a +1 until after a 'query-fds' - it is not until the monitor has reconnected and learned what fds it should be aware of that incrementing the refcount again makes sense. But that would mean making 'query-fds' track whether this is the first call since the monitor reconnected, as it shouldn't normally increase refcounts.
This doesn't sound ideal.
The other alternative is that the monitor never re-increments a refcount. Once a monitor disconnects, that fd is lost to the monitor, and a reconnected monitor must pass in a new fd to be re-associated with the fdset. In other words, the monitor's use of an fd is a one-way operation, starting life in use but ending at the first disconnect or remove-fd.
I would vote for this 2nd alternative. As long as we're not introducing an fd leak. And I don't think we are if we decrement the refcount on remove-fd or on QMP disconnect.
1. client calls 'add-fd', qemu is now tracking fd=4 with refcount 1, in use by monitor, as member of fdset1 2. client calls 'device-add' with /dev/fdset/1 as the backing filename, so qemu_open() increments the refcount to 2 3. client crashes, so all tracked fds are visited; fd=4 had not yet been passed to 'remove-fd', so qemu decrements refcount to 1, but leaves fd=4 open because it is still in use by the block device 4. client re-establishes QMP connection, and 'query-fds' lets client learn about fd=4 still being open as part of fdset1, but also informs client that fd is not in use by the monitorAnd in step 4 the QMP connection will increment the refcount +1 for all fds that persisted through the QMP disconnect. (?)I'm not sure we need the refcount increment on reconnect. 'query-fds' should provide enough information for the new monitor to know what fdsets are still in use by qemu, even though they are no longer available to 'remove-fd' from the monitor, and if the monitor is worried about keeping the fd set alive it can call 'add-fd' to again associate a new fd with the set. The lifetime of a set is thus as long as any of its associated fds have a non-zero refcount.
This sounds good to me.And qemu_open will need to make sure the monitor in_use flag is true before dup'ing an fd.
-- Regards, Corey
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