Re: [PATCH 1/3] picolcd: driver for PicoLCD HID device

On Thu, Feb 25, 2010 at 08:34:42AM -0700, Rick L. Vinyard, Jr. wrote:

Jiri Kosina wrote:

quoted

On Wed, 24 Feb 2010, Rick L. Vinyard, Jr. wrote:

quoted

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