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From: David Brownell <david-b@pacbell.net>
To: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [linux-usb-devel] device struct bloat
Date: Mon, 5 Nov 2007 17:38:34 -0800
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Cc: linux-usb-devel@lists.sourceforge.net, Greg KH <greg@kroah.com>,
       Alan Stern <stern@rowland.harvard.edu>, Oliver Neukum <oneukum@suse.de>,
       Ingo Molnar <mingo@elte.hu>,
       Stephen Hemminger <shemminger@linux-foundation.org>,
       linux-kernel@vger.kernel.org, apw <apw@shadowen.org>
References: <20071103124823.6059640e@shemminger-laptop> <1194260234.27652.425.camel@twins> <20071105224929.GA30521@kroah.com>
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On Monday 05 November 2007, Greg KH wrote:
> --- linux-2.6-2.orig/drivers/spi/spi.c
> +++ linux-2.6-2/drivers/spi/spi.c

It'd be quicker to end up in the right hands if you had
split this big and random patch according to subsystem...

There's already a patch in the MM queue that removes
the SPI-private semaphore.  Except that it's missing
the bug noted below.

The class semaphore removal would be a different issue.


> @@ -613,7 +613,7 @@ int spi_write_then_read(struct spi_devic
> ????????}
> ?
> ????????/* ... unless someone else is using the pre-allocated buffer */
> -???????if (down_trylock(&lock)) {
> +???????if (mutex_trylock(&lock)) {

According to its kerneldoc, mutex_trylock() follows the
spinlock model not the semaphore model.  So the sense of
this test is incorrect ... as will be any similar changes
in other parts of this patch:

 * NOTE: this function follows the spin_trylock() convention, so
 * it is negated to the down_trylock() return values! Be careful
 * about this when converting semaphore users to mutexes.

So the patch in the MM queue says "if (!mutex_trylock(...)) {

> ????????????????local_buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);
> ????????????????if (!local_buf)
> ????????????????????????return -ENOMEM;

-
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