linux-kernel - Re: [GIT, RFC] Killing the Big Kernel Lock II

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Date:	Mon, 29 Mar 2010 14:30:28 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Matthew Wilcox <matthew@....cx>,
	Thomas Gleixner <tglx@...utronix.de>, jblunck@...e.de,
	Alan Cox <alan@...ux.intel.com>, Ingo Molnar <mingo@...e.hu>,
	gregkh@...e.de
Subject: Re: [GIT, RFC] Killing the Big Kernel Lock II

Arnd Bergmann <arnd@...db.de> writes:

>> - The seek function in uhci-debug.c probably is still racy.
>
> That function could be removed in favor of using generic_file_ioctl
> and setting i_size to up->size.

Does that lock against read in libfs? 

> Also, the race is only between concurrent calls of llseek on
> the same file descriptor, which is undefined anyway.
> The current code also doesn't protect you against partial updates
> of f_pos during ->read() on 32 bit systems (nothing ever does),

That is not what I meant.

> and it even fails to protect against the concurrent llseek race
> because the assignment is done outside of the f_pos update.

I wasn't sure it would protect against parallel reads.

Does it?

> The patch looks correct, but I probably wouldn't bother with the rename,
> and simply drop the BKL in the caller.

I think a rename is better, I take compile errors over subtle
breakage any day.

>>Author: Andi Kleen <ak@...ux.intel.com>
>>Date:   Mon Mar 29 01:15:32 2010 +0200
>>
>>    USB-BKL: Remove BKL from usb serial drivers ioctl handlers
>>
>>    I audited all the low level serial ioctl handlers and none of them
>>    actually need the BKL.
>>
>>    To make sure all code is checked change the usb_serial_driver ->ioctl
>> field to ->unlocked_ioctl
>>
>>    Note this is still called for now with BKL held because tty drivers
>>    don't have a ->unlocked_ioctl from the tty layer in mainline.
>>    This could be trivially changed now though.
>>
>>    Signed-off-by: Andi Kleen <ak@...ux.intel.com>
>
> The serial_ioctl function is already called without the BKL, depite the
> name. tty_operations->ioctl was converted a long time ago, so I guess this
> patch can be dropped from your series.

Ok.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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