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Re: [PATCH 1/4] block/nbd: fix drain dead-lock because of nbd reconnect-

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH 1/4] block/nbd: fix drain dead-lock because of nbd reconnect-delay
Date: Wed, 3 Feb 2021 17:44:34 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0

03.02.2021 17:21, Roman Kagan wrote:
On Wed, Feb 03, 2021 at 04:10:41PM +0300, Vladimir Sementsov-Ogievskiy wrote:
03.02.2021 13:53, Roman Kagan wrote:
On Thu, Sep 03, 2020 at 10:02:58PM +0300, Vladimir Sementsov-Ogievskiy wrote:
We pause reconnect process during drained section. So, if we have some
requests, waiting for reconnect we should cancel them, otherwise they
deadlock the drained section.

How to reproduce:

1. Create an image:
     qemu-img create -f qcow2 xx 100M

2. Start NBD server:
     qemu-nbd xx

3. Start vm with second nbd disk on node2, like this:

    ./build/x86_64-softmmu/qemu-system-x86_64 -nodefaults -drive \
       file=/work/images/cent7.qcow2 -drive \
       -vnc :0 -m 2G -enable-kvm -vga std

4. Access the vm through vnc (or some other way?), and check that NBD
     drive works:

     dd if=/dev/sdb of=/dev/null bs=1M count=10

     - the command should succeed.

5. Now, kill the nbd server, and run dd in the guest again:

     dd if=/dev/sdb of=/dev/null bs=1M count=10

Now Qemu is trying to reconnect, and dd-generated requests are waiting
for the connection (they will wait up to 60 seconds (see
reconnect-delay option above) and than fail). But suddenly, vm may
totally hang in the deadlock. You may need to increase reconnect-delay
period to catch the dead-lock.

VM doesn't respond because drain dead-lock happens in cpu thread with
global mutex taken. That's not good thing by itself and is not fixed
by this commit (true way is using iothreads). Still this commit fixes
drain dead-lock itself.

Note: probably, we can instead continue to reconnect during drained
section. To achieve this, we may move negotiation to the connect thread
to make it independent of bs aio context. But expanding drained section
doesn't seem good anyway. So, let's now fix the bug the simplest way.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
   block/nbd.c | 5 +++++
   1 file changed, 5 insertions(+)

diff --git a/block/nbd.c b/block/nbd.c
index 9daf003bea..912ea27be7 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -242,6 +242,11 @@ static void coroutine_fn 
nbd_client_co_drain_begin(BlockDriverState *bs)
       nbd_co_establish_connection_cancel(bs, false);
+    if (s->state == NBD_CLIENT_CONNECTING_WAIT) {
+        qemu_co_queue_restart_all(&s->free_sema);
+    }
   static void coroutine_fn nbd_client_co_drain_end(BlockDriverState *bs)

This basically defeats the whole purpose of reconnect: if the nbd client
is trying to reconnect, drain effectively cancels that and makes all
in-flight requests to complete with an error.

I'm not suggesting to revert this patch (it's now in the tree as commit
8c517de24a), because the deadlock is no better, but I'm afraid the only
real fix is to implement reconnect during the drain section.  I'm still
trying to get my head around it so no patch yet, but I just wanted to
bring this up in case anybody beats me to it.

What do you mean by "reconnect during drained section"? Trying to
establish the connection? Or keeping in-flight requests instead of
cancelling them? We can't keep in-flight requests during drained
section, as it's the whole sense of drained-section: no in-flight
requests. So we'll have to wait for them at drain_begin (waiting up to
reconnect-delay, which doesn't seem good and triggers dead-lock for
non-iothread environment) or just cancel them..

Do you argue that waiting on drained-begin is somehow better than

Sorry I should have used more accurate wording to be clear.

Yes, my point is that canceling the requests on entry to a drained
section is incorrect.  And no, it doesn't matter if it can be long:
that's the price you pay for doing the drain.  Think of reconnect as a
special case of a slow connection: if an nbd reply from the server is
delayed for whatever reason without dropping the connection, you have to
wait here, too.  (In fact, reconnect is even slightly better here since
it has a configurable finite timeout while the message delay does not

Does it make sense?

Hmm, yes..

But then we should fix the original deadlock some other way.. Probably it will
be possible only by using iothread for nbd node (I failed to find original
thread where someone said that iothreads is a solution). And than we can do 
in nbd_client_co_drain_begin() only if bs doesn't have a separate iothread.

Or, the problem is that after drained section (assume it was short) we
are in NBD_CLIENT_CONNECTING_NOWAIT ?  Fixing it should be simple
enough, we just need to check the time at drained_end and if the delay
is not expired enable NBD_CLIENT_CONNECTING_WAIT agaub,,

Hmm, I didn't get thus far but this is probably an issue too.


Best regards,

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