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Re: [PATCH] Use atomic_t and wait_event to track dquot pincount

To: xfs@xxxxxxxxxxx, xfs-dev@xxxxxxx
Subject: Re: [PATCH] Use atomic_t and wait_event to track dquot pincount
From: Peter Leckie <pleckie@xxxxxxx>
Date: Wed, 24 Sep 2008 16:53:22 +1000
In-reply-to: <20080924060508.GH5448@disturbed>
References: <48D9C1DD.6030607@xxxxxxx> <20080924060508.GH5448@disturbed>
User-agent: Thunderbird (X11/20070305)
Dave Chinner wrote:
[ Pete, please wrap your text at 68-72 columns]
Ok will do in future
On Wed, Sep 24, 2008 at 02:28:13PM +1000, Peter Leckie wrote:
The reason the lsn had changed was due to it not being initialized
by  the time a copy of the lsn was taken in xfs_qm_dqflush().
The lsn was then latter updated causing the test  in
xfs_qm_dqflush_done() to fail.

Synchronizations between the 2 functions is done by the pincount
in  struct xfs_dquot_t  and xfs_qm_dqflush() calls
xfs_qm_dqunpin_wait() to wait for the pincount == 0. However after
been woken up it does not validate the pincount is actually 0,

Sure - that's because we only ever send a wakeup when the pin count
falls to zero. Because we have to be holding the dquot lock when we
either pin a dquot or wait for it to be unpinned, the act of waiting
for it to be unpinned with the dquot lock held guarantees that it
is not pinned when we wake up.

IOWs, the pin count cannot be incremented while we are waiting for
it to be unpinned, and hence it must be zero when we are woken......

allowing a false wake up by the scheduler to  cause
xfs_qm_dqflush() to prematurely start processing the dquot.

.... which means I can't see how that would happen...

What am I missing here?
Yeah it's quite intriguing  isn't it, I added the pincount to the
dquot tracing code and sure enough it's 1 all the way through
xfs_qm_dqflush() I can tell you that none of the XFS code is
causing it to wake up. I was going to add some tracing code to
the scheduler to determine who is causing us to wake up however
I had other priorities to work on.

Reading the code it appears things like a threads dying or a
system suspending can cause it. However none of these had
happened, after reading other examples in the Linux kernel
it appeared pretty standard to always re-evaluate the condition
after being woken so I suspect that Linux simply does not
guarantee that we will only be woken by the thread calling
wake_up on  us.

Any insight into this would be much appreciated as I'm also very curious
as to why this is happening.
As to the patch, your mailer has whitespace damaged it so you need
to be fix that up.

Ahh yeah copy and past from a terminal, I'll  fix up the patch and
re-send shortly,

Thanks Dave.


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