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From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
To: "Rick L. Vinyard, Jr." <rvinyard@cs.nmsu.edu>
Cc: "Jiri Kosina" <jkosina@suse.cz>,
	"Oliver Neukum" <oliver@neukum.org>,
	"Bruno Prémont" <bonbons@linux-vserver.org>,
	linux-input@vger.kernel.org, linux-usb@vger.kernel.org,
	linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	"Nicu Pavel" <npavel@ituner.com>
Subject: Re: [PATCH 1/3] picolcd: driver for PicoLCD HID device
Date: Fri, 26 Feb 2010 00:12:17 -0800	[thread overview]
Message-ID: <20100226081217.GA17444@core.coreip.homeip.net> (raw)
In-Reply-To: <7316226fbd1cb53b7f90965c54acd343.squirrel@intranet.cs.nmsu.edu>

On Thu, Feb 25, 2010 at 08:34:42AM -0700, Rick L. Vinyard, Jr. wrote:
> Jiri Kosina wrote:
> > On Wed, 24 Feb 2010, Rick L. Vinyard, Jr. wrote:
> >
> >> The key difference is the replacement of spin_lock() with spin_trylock()
> >> such that if the non-interrupt code has already obtained the lock, the
> >> interrupt will not deadlock but instead take the else path and schedule
> >> a
> >> framebuffer update at the next interval.
> >
> > Why is _irqsave() and/or deferred work not enough? The aproach with
> > _trylock() seems to be overly complicated for no good reason (I personally
> > become very suspicious every time I see code that is using _trylock()).
> >
> 
> I was concerned about _irqsave() because the lock is split across two
> functions to protect the urb after it is handed off to the usb subsystem
> with usb_submit_urb(). It's locked in g13_fb_send() and unlocked in the
> urb completion callback.
> 
> As for deferred work, the g13_fb_send() is the I/O portion of the deferred
> framebuffer callback. I was concerned that without a lock one deferred
> update could hand the urb off to the usb subsystem and a second could try
> to access it before it was handed back to the driver.
> 
> In this case the _trylock() would fail and in the else patch we would
> defer yet again until the next update cycle.
> 

What you are looking for here is called test_and_set_bit(). Do not muddy
the waters with a lock.

-- 
Dmitry

WARNING: multiple messages have this Message-ID (diff)
From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
To: "Rick L. Vinyard, Jr." <rvinyard@cs.nmsu.edu>
Cc: "Jiri Kosina" <jkosina@suse.cz>,
	"Oliver Neukum" <oliver@neukum.org>,
	"Bruno Prémont" <bonbons@linux-vserver.org>,
	linux-input@vger.kernel.org, linux-usb@vger.kernel.org,
	linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	"Nicu Pavel" <npavel@ituner.com>
Subject: Re: [PATCH 1/3] picolcd: driver for PicoLCD HID device
Date: Fri, 26 Feb 2010 08:12:17 +0000	[thread overview]
Message-ID: <20100226081217.GA17444@core.coreip.homeip.net> (raw)
In-Reply-To: <7316226fbd1cb53b7f90965c54acd343.squirrel@intranet.cs.nmsu.edu>

On Thu, Feb 25, 2010 at 08:34:42AM -0700, Rick L. Vinyard, Jr. wrote:
> Jiri Kosina wrote:
> > On Wed, 24 Feb 2010, Rick L. Vinyard, Jr. wrote:
> >
> >> The key difference is the replacement of spin_lock() with spin_trylock()
> >> such that if the non-interrupt code has already obtained the lock, the
> >> interrupt will not deadlock but instead take the else path and schedule
> >> a
> >> framebuffer update at the next interval.
> >
> > Why is _irqsave() and/or deferred work not enough? The aproach with
> > _trylock() seems to be overly complicated for no good reason (I personally
> > become very suspicious every time I see code that is using _trylock()).
> >
> 
> I was concerned about _irqsave() because the lock is split across two
> functions to protect the urb after it is handed off to the usb subsystem
> with usb_submit_urb(). It's locked in g13_fb_send() and unlocked in the
> urb completion callback.
> 
> As for deferred work, the g13_fb_send() is the I/O portion of the deferred
> framebuffer callback. I was concerned that without a lock one deferred
> update could hand the urb off to the usb subsystem and a second could try
> to access it before it was handed back to the driver.
> 
> In this case the _trylock() would fail and in the else patch we would
> defer yet again until the next update cycle.
> 

What you are looking for here is called test_and_set_bit(). Do not muddy
the waters with a lock.

-- 
Dmitry

  reply	other threads:[~2010-02-26  8:12 UTC|newest]

Thread overview: 81+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2010-02-20 23:10 [Patch 0/3] backlight Bruno Prémont
2010-02-20 23:10 ` Bruno Prémont
2010-02-20 23:18 ` [PATCH 2/3] backlight: mark struct backlight_ops const Bruno Prémont
2010-02-20 23:18   ` Bruno Prémont
2010-02-20 23:18   ` Bruno Prémont
2010-02-20 23:18   ` Bruno Prémont
     [not found]   ` <20100221001851.673db863-hY15tx4IgV39zxVx7UNMDg@public.gmane.org>
2010-02-22 19:35     ` Mike Frysinger
2010-02-22 19:35       ` Mike Frysinger
2010-02-22 19:35       ` Mike Frysinger
2010-02-22 19:35       ` Mike Frysinger
2010-02-26 11:56   ` [PATCH] " Bruno Prémont
2010-02-26 11:56     ` Bruno Prémont
2010-02-20 23:20 ` [PATCH 1/3] backlight: Add backlight_device parameter to check_fb Bruno Prémont
2010-02-20 23:20   ` Bruno Prémont
2010-02-24 16:00   ` [PATCH 1/3] picolcd: driver for PicoLCD HID device Bruno Prémont
2010-02-24 16:00     ` Bruno Prémont
2010-02-24 16:00     ` Bruno Prémont
2010-02-24 16:01     ` [PATCH 2/3] hid: add suspend/resume hooks for hid drivers Bruno Prémont
2010-02-25  4:19       ` Oliver Neukum
2010-02-25 10:12         ` Bruno Prémont
2010-02-24 16:01     ` [PATCH 3/3] hid-picolcd: make use of new suspend/resume hooks Bruno Prémont
2010-02-24 16:01       ` Bruno Prémont
2010-02-24 18:27     ` [PATCH 1/3] picolcd: driver for PicoLCD HID device Oliver Neukum
2010-02-24 18:27       ` Oliver Neukum
2010-02-24 18:27       ` Oliver Neukum
2010-02-24 21:44       ` Rick L. Vinyard, Jr.
2010-02-24 21:44         ` Rick L. Vinyard, Jr.
2010-02-24 21:44         ` Rick L. Vinyard, Jr.
2010-02-25  4:11         ` Oliver Neukum
2010-02-25  4:11           ` Oliver Neukum
2010-02-25 11:00         ` Jiri Kosina
2010-02-25 11:00           ` Jiri Kosina
2010-02-25 15:34           ` Rick L. Vinyard, Jr.
2010-02-25 15:34             ` Rick L. Vinyard, Jr.
2010-02-26  8:12             ` Dmitry Torokhov [this message]
2010-02-26  8:12               ` Dmitry Torokhov
2010-02-25 11:07     ` Jiri Kosina
2010-02-25 11:07       ` Jiri Kosina
2010-02-25 11:07       ` Jiri Kosina
2010-02-25 11:32       ` Bruno Prémont
2010-02-25 11:32         ` Bruno Prémont
2010-02-25 11:32         ` Bruno Prémont
2010-02-25 15:18         ` Jiri Kosina
2010-02-25 15:18           ` Jiri Kosina
2010-02-25 15:18           ` Jiri Kosina
2010-02-25 15:29           ` Bruno Prémont
2010-02-25 15:29             ` Bruno Prémont
2010-03-13 19:39           ` Bruno Prémont
2010-03-13 19:39             ` Bruno Prémont
2010-03-13 21:35             ` Alan Stern
2010-03-13 21:35               ` Alan Stern
2010-03-13 22:13               ` [PATCH] hid: Register debugfs entries before adding device Bruno Prémont
2010-03-13 22:13                 ` Bruno Prémont
2010-03-15 13:48                 ` Jiri Kosina
2010-02-25 17:52         ` [PATCH 1/3] picolcd: driver for PicoLCD HID device Rick L. Vinyard, Jr.
2010-02-25 17:52           ` Rick L. Vinyard, Jr.
2010-02-25 17:52           ` Rick L. Vinyard, Jr.
2010-02-26  8:15       ` Dmitry Torokhov
2010-02-26  8:15         ` Dmitry Torokhov
2010-02-26  8:15         ` Dmitry Torokhov
2010-03-03  6:04     ` Pavel Machek
2010-03-03  6:04       ` Pavel Machek
2010-02-26 11:53   ` [PATCH] backlight: Add backlight_device parameter to check_fb Bruno Prémont
2010-02-26 11:53     ` Bruno Prémont
2010-02-20 23:28 ` [PATCH 3/3] backlight: fix missing/incomplete registration failure handling Bruno Prémont
2010-02-20 23:28   ` [PATCH 3/3] backlight: fix missing/incomplete registration failure Bruno Prémont
2010-02-21  8:04   ` [PATCH 3/3] backlight: fix missing/incomplete registration failure handling Harald Welte
2010-02-21 13:35   ` Thadeu Lima de Souza Cascardo
2010-02-21 13:35     ` [PATCH 3/3] backlight: fix missing/incomplete registration Thadeu Lima de Souza Cascardo
2010-02-24 15:33     ` [PATCH 3/3] backlight: fix missing/incomplete registration failure handling Anisse Astier
2010-02-24 15:33       ` [PATCH 3/3] backlight: fix missing/incomplete registration Anisse Astier
2010-02-26 11:59   ` [PATCH] backlight, classmate-laptop: fix missing registration failure handling Bruno Prémont
2010-02-26 16:20     ` Thadeu Lima de Souza Cascardo
2010-02-26 16:32       ` Matthew Garrett
2010-02-26 16:45         ` Richard Purdie
2010-02-26 22:25           ` Bruno Prémont
2010-02-26 12:02   ` [PATCH] backlight, appledisplay: fix incomplete " Bruno Prémont
2010-02-26 12:04   ` [PATCH] backlight, blackfin: fix missing " Bruno Prémont
2010-02-26 12:04     ` [PATCH] backlight, blackfin: fix missing registration failure Bruno Prémont
2010-02-26 12:17   ` [PATCH] backlight, msi-laptop, msi-wmi: fix incomplete registration failure handling Bruno Prémont
2010-02-26 12:20   ` [PATCH] backlight, panasonic-laptop: " Bruno Prémont

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