Re: [PATCH v8 21/22] counter: 104-quad-8: Replace mutex with spinlock

On Tue, Feb 16, 2021 at 07:27:20PM +0000, Jonathan Cameron wrote:

On Tue, 16 Feb 2021 11:16:03 +0900
William Breathitt Gray [off-list ref] wrote:

quoted

On Sun, Feb 14, 2021 at 06:19:46PM +0000, Jonathan Cameron wrote:

quoted

On Fri, 12 Feb 2021 21:13:45 +0900
William Breathitt Gray [off-list ref] wrote:
  

quoted

This patch replaces the mutex I/O lock with a spinlock. This is in
preparation for a subsequent patch adding IRQ support for 104-QUAD-8
devices; we can't sleep in an interrupt context, so we'll need to use a
spinlock instead.

Cc: Syed Nayyar Waris <redacted>
Signed-off-by: William Breathitt Gray <redacted>  

Why do these need to be raw_spin_locks?
Normally only need to do that if in code related to interrupt chips etc,
not their use.

Jonathan  

This lock can be taken in an interrupt context in a subsequent patch:
counter_push_event() called by quad8_irq_handler() can end up calling
the Counter component callbacks which take this lock. We can't use a
mutex nor a regular spinlock because those can sleep on RT setups [1]

But on RT setups the interrupts become threads and are preemptable etc,
so that shouldn't matter.  There are a few corner cases that cause
trouble, but in most normal drivers you should be fine with a
spin_lock.

Jonathan

Thinking this over again I realize you're probably right. A simple
spin_lock should be fine for these drivers, so I'll switch over to that
instead of the raw_spin_lock for this driver and the Counter interface
code.

William Breathitt Gray

quoted

which would result in a deadlock due to the interrupt context here -- so
therefore we're left with using raw_spin_lock.

[1] https://lwn.net/Articles/367219/

William Breathitt Gray