MTX_DEF versus MTX_SPIN

[Julian Elischer]

On 11/3/10 10:52 AM, Julian Elischer wrote:
> On 11/3/10 10:17 AM, John Baldwin wrote:
>> On Wednesday, November 03, 2010 1:04:13 pm mdf at freebsd.org wrote:
>>>
>>> So a MTX_DEF is okay in that environment?
>> Yes.  In fact, the reason to have threads for interrupt handlers is 
>> to allow
>> interrupt handlers to use non-spin locks that block when the lock 
>> is held.
>>
>> MTX_SPIN locks are generally not needed in device drivers.  The 
>> only reason a
>> driver would use one is if it used a filter handler which does not 
>> run in a
>> threaded context.
>
oops
a line got deleted I think..
> It should be noted that in the case where you really just want to 
> spin a few
> instructions because some other thread is accessing a structure you 
> want,
  ... then the BTX_DEF code will spin for a short while before ...
> descheduling you. so you don't always incur the scheduling overhead.
>
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