linux-kernel - Re: [PATCH] usb: f_mass_storage: test whether thread is running before starting another

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Date:	Tue, 05 Apr 2016 20:07:45 +0200
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>,
	Tony Lindgren <tony@...mide.com>, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org,
	Felipe Balbi <felipe.balbi@...ux.intel.com>,
	Bin Liu <b-liu@...com>, pali.rohar@...il.com
Subject: Re: [PATCH] usb: f_mass_storage: test whether thread is running before starting another

> On Tue, 5 Apr 2016, Michal Nazarewicz wrote:
>> When binding the function to usb_configuration, check whether the thread
>> is running before starting another one.  Without that, when function
>> instance is added to multiple configurations, fsg_bing starts multiple
>> threads with all but the latest one being forgotten by the driver.  This
>> leads to obvious thread leaks, possible lockups when trying to halt the
>> machine and possible more issues.
>> 
>> This fixes issues with legacy/multi¹ gadget as well as configfs gadgets
>> when mass_storage function is added to multiple configurations.
>> 
>> This change also simplifies API since the legacy gadgets no longer need
>> to worry about starting the thread by themselves (which was where bug
>> in legacy/multi was in the first place).
>> 
>> ¹ I have no example failure though.  Conclusion that legacy/multi has
>>   a bug is based purely on me reading the code.
>> 
>> Signed-off-by: Michal Nazarewicz <mina86@...a86.com>

On Tue, Apr 05 2016, Alan Stern wrote:
> This doesn't address the problem I raised in a previous email.  
> Sharing one thread among several function instances in the same config
> will not work if one of them encounters an error.

Each usb_function_instance has its own fsg_common and its own thread.
This was true in the past and is true with this patch as well.

And unless I’m missing something, sharing a thread among multiple
usb_function’s does not prevent the driver from working correctly.

Having the thread run even when it’s not used may be considered wasteful
but that’s an orthogonal issue to the configfs failure.

-- 
Best regards
ミハウ “𝓶𝓲𝓷𝓪86” ナザレヴイツ
«If at first you don’t succeed, give up skydiving»