[Drbd-dev] DRBD8: Deadlock in PausedSyncS
Ernest.Montrose at stratus.com
Thu Aug 2 13:58:47 CEST 2007
Yeah, I agree. There are a few more situations other then this one.
I will try to actually test a few of those with this patch and let you
know if I find anything weird. Thanks!
From: Philipp Reisner [mailto:philipp.reisner at linbit.com]
Sent: Thursday, August 02, 2007 5:45 AM
To: drbd-dev at linbit.com
Cc: Montrose, Ernest
Subject: Re: [Drbd-dev] DRBD8: Deadlock in PausedSyncS
On Tuesday 31 July 2007 00:31:19 Montrose, Ernest wrote:
> Hi all,
> We are seeing a problem where we deadlock if a pause sync request
> happens while attaching. Below is an explaination of what I think is
> Consider two nodes X primary and Y Secondary.
> 1. X becomes Secondary/Diskless
> 2. Y becomes Primary
> 3. X tries to Attach and sends its states/uuids to Y
> 4. While Y is in receive_state() doing a drbd_sync_handshake(), it
> receives a Paused Sync request.
> This is where the trouble starts.
> 5. aftr_isp is changed from 0->1 on Y and after_state_ch() called.
> a drbd_send_state() from Y.
> 6. X receives States from Y but no uuids and runs a
> drbd_sync_handshake() with the old uuids and
> we deadlocked with PausedSyncS on both sides.
> I am not sure how to best fix this. Perhaps we should not call
> in after_state_ch() for a sync request from the peer if the peer's
> is diskless.
> Or we do send states, sends the uuids as well. The attached patch
> at least serve as an illustration of the issue if not the correct fix.
Your patch was correct I think. But in reality we have not only this
one case of the problem it is a whole class of such problems.
While the node that gets the new disk is in disk=Negotiating, it
will run drbd_sync_handshake() on each state packet that comes in.
We need to avoid this. Calling pause-sync is just one possible way
to cause the transmittion of a state packets, this is a whole class
of such problems.
Instead of fixing every place where we send a state packet, to not
send it if the peer has no disk, I decided to fix the receiving
: Dipl-Ing Philipp Reisner Tel +43-1-8178292-50 :
: LINBIT Information Technologies GmbH Fax +43-1-8178292-82 :
: Vivenotgasse 48, 1120 Vienna, Austria http://www.linbit.com :
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