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From:   madhuparnabhowmik10@gmail.com
To:     gregkh@linuxfoundation.org, hariprasad.kelam@gmail.com,
        colin.king@canonical.com, tony.olech@elandigitalsystems.com
Cc:     linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        andrianov@ispras.ru
Subject: Possible data-race related bug in u132_hcd module.
Date:   Mon, 30 Mar 2020 17:22:43 +0530
Message-Id: <20200330115243.11107-1-madhuparnabhowmik10@gmail.com>
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Sender: linux-kernel-owner@vger.kernel.org
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Hi,

This bug is found by  Linux Driver Verification project (linuxtesting.org).

The bug is related to the parallel execution of u132_probe() function and u132_hcd_exit() function in u132_hcd.c. In case the module is unloaded when the probe function is executing there can be data race as the mutex lock u132_module_lock is not used properly. 

i) Usage of mutex lock only when writing into the u132_exiting variable in u132_hcd_exit(). The lock is not used when this variable is read in u132_probe().

Moreover, this variable does not serve its purpose, as even if locking is used while the u132_exiting variable is read in probe(), the function may still miss that exit function is executing if it acquires the mutex before exit() function does.

How to fix this?

ii) Usage of mutex while adding entries in u132_static_list in probe function but not in exit function while unregistering.
This should be easy to fix by holding the mutex in the exit function as well.

There can be other synchronization problems related to the usage of u132_module_lock in this module, I have only spotted these so far.

Please let me know if this bug report is helpful and I can send a patch fixing it.

Thank you,
Madhuparna