spinlock semantics

Nuno Antunes nuno.antunes at gmail.com
Sun Feb 17 11:13:18 PST 2008

On Feb 17, 2008 6:52 PM, Matthew Dillon <dillon at apollo.backplane.com> wrote:
> :Hi,
> :
> :While reading about DragonFlys locking primitives in a quest to try to
> :understand which kind of lock best applies in each situation, I found
> :the following at the spinlock(9) manpage.
> :
> :"If you wish to disable FAST interrupts and IPIs you need to enter a
> :critical section prior to obtaining the spinlock."
> :
> :But then, on the mailing list archive, I found a message [1] where one
> :can read the following with regards to spinlocks:
> :
> :"Automatically enters a critical section."
> :
> :So, naturally, my question is which statement is true? Looking at the
> :implementation, i don't see any calls to crit_* there, so I assume
> :that the manpage is correct.
> :
> :Thanks,
> :Nuno
> :
> :[1] http://leaf.dragonflybsd.org/mailarchive/kernel/2006-11/msg00026.html
>     spinlocks increment and decrement mycpu->gd_spinlocks_rd/wr.
>     This prevents thread preemption from occuring (lwkt_thread.c line 846).
>     This means that a FAST interrupt can still operate, and any threaded
>     interrupt (which is basically all interrupts except the clock interrupt)
>     will still be scheduled, but will NOT be able to preempt the current
>     thread.
>     It's kinda like a poor-man's critical section.  It has a few issues
>     the main one being that it does not currently check to see if a yield
>     is needed after the last spinlock is released.  I wanted the spinlock
>     path to be as fast as possible.
>     A critical section is a more encompassing feature.  A critical section
>     will cause the interrupt itself to be deferred if it occurs on the
>     cpu in question... for example, an interrupt which schedules a
>     thread during a spinlock will not preempt, but an interrupt which occurs
>     during a critical section won't even schedule the interrupt thread in
>     the first place.  It will just mark the interrupt as pending in the
>     machine-dependant portion of the globaldata structure, mask the
>     interrupt, and return.
>     Thus the scheduler itself can use a critical section to protect against
>     reentrancy.
>                                         -Matt
>                                         Matthew Dillon
>                                         <dillon at backplane.com>

I understand now, more than what I asked about. :) Thanks for the explanation.


PS: maybe i should update the manpage...

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