linux-kernel - Re: [PATCH 1/2] xen: xenbus_dev_frontend: Fix XS_TRANSACTION

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Message-ID: <1d10edc6-8ad6-bc58-432c-d1867f0ab57a@invisiblethingslab.com>
Date:   Tue, 20 Feb 2018 04:56:00 +0000
From:   Simon Gaiser <simon@...isiblethingslab.com>
To:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org
Cc:     stable@...r.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] xen: xenbus_dev_frontend: Fix XS_TRANSACTION_END
 handling

Juergen Gross:
> 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.

So if xenstore frees the transaction why should we keep it in the list
with pending transaction in xenbus_dev_frontend? That's exactly what
this patch fixes by always removing it from the list, not only on a
successful response (See below for the EINVAL case).

[...]
>> 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.

With "considering a transaction closed" I mean "take the code path which
removes the transaction from the list with pending transactions".

From the follow-up mail:
>>> 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.
> 
> Sorry, I stand corrected: the ENOENT case should never happen, as this
> case is tested in xenbus_write_transaction(). It doesn't hurt to test
> for ENOENT, though.
> 
> What should be handled is EINVAL: this would happen if a user specified
> a string different from "T" and "F".

Ok, I will handle those cases in xs_request_exit(). Although I don't
like that this depends on the internals of xenstore (At least to me it's
not obvious why it should only return ENOENT or EINVAL in this case).

In the xenbus_write_transaction() case checking the string before
sending the transaction (like the transaction itself is verified) would
avoid this problem.



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