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Subject: Re: [PATCH 1/2] xen: xenbus_dev_frontend: Fix XS_TRANSACTION_END
 handling
To:     Simon Gaiser <simon@invisiblethingslab.com>,
        xen-devel@lists.xenproject.org
Cc:     stable@vger.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        linux-kernel@vger.kernel.org
References: <20180207222236.7434-1-simon@invisiblethingslab.com>
From:   Juergen Gross <jgross@suse.com>
Message-ID: <ea0630bd-5139-5f79-847c-f88f733b5053@suse.com>
Date:   Mon, 12 Feb 2018 09:49:26 +0100
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On 07/02/18 23:22, Simon Gaiser wrote:
> Commit fd8aa9095a95 ("xen: optimize xenbus driver for multiple
> concurrent xenstore accesses") made a subtle change to the semantic of
> xenbus_dev_request_and_reply() and xenbus_transaction_end().
> 
> Before on an error response to XS_TRANSACTION_END
> xenbus_dev_request_and_reply() would not decrement the active
> transaction counter. But xenbus_transaction_end() has always counted the
> transaction as finished regardless of the response.

Which is correct now. Xenstore will free all transaction related
data regardless of the response. A once failed transaction can't
be repaired, it has to be repeated completely.

The real problem is decrementing the counter when XS_TRANSACTION_END
for a non-existing transaction is being sent.

> The new behavior is that xenbus_dev_request_and_reply() and
> xenbus_transaction_end() will always count the transaction as finished
> regardless the response code (handled in xs_request_exit()).

ENOENT should not decrement the transaction counter, while all
other responses to XS_TRANSACTION_END should still do so.

> But xenbus_dev_frontend tries to end a transaction on closing of the
> device if the XS_TRANSACTION_END failed before. Trying to close the
> transaction twice corrupts the reference count. So fix this by also
> considering a transaction closed if we have sent XS_TRANSACTION_END once
> regardless of the return code.

A transaction in the list of transactions should not considered to be
finished. Either it is not on the list or it is still pending.

> 
> Cc: <stable@vger.kernel.org> # 4.11
> Fixes: fd8aa9095a95 ("xen: optimize xenbus driver for multiple concurrent xenstore accesses")
> Signed-off-by: Simon Gaiser <simon@invisiblethingslab.com>

So: your patch is a band-aid trying to cure the symptoms, but not the
real problem. Please do it properly.

NAK.


Juergen