Re: recursive spinlocks. Shoot.

[Kevin O'Connor]

On Sun, May 18, 2003 at 07:24:17PM +0200, Peter T. Breuer wrote:
> Though I've got quite good at finding and removing deadlocks in my old
> age, there are still two popular ways that the rest of the world's
> prgrammers often shoot themselves in the foot with a spinlock:
> 
>    a) sleeping while holding the spinlock
>    b) taking the spinlock in a subroutine while you already have it
[...]
> The second method is used by programmers who aren't aware that some
> obscure subroutine takes a spinlock, and who recklessly take a lock
> before calling a subroutine (the very thought sends shivers down my
> spine ...).

Recursive spinlocks only hide the problem - consider programmers who take
lock B and recklessly call a subroutine that takes lock A followed be lock
B.  The resulting code will appear to work fine, but may have introduced a
subtle AB-BA deadlock.  I'd rather have a coding defect that reliably and
consistently causes a deadlock than one that causes deadlocks in rare
timing related cases.

>A popular scenario involves not /knowing/ that your routine
> is called by the kernel with some obscure lock already held, and then
> calling a subroutine that calls the same obscure lock.

If the kernel invokes a callback with an obscure lock held (that is
promiscuous enough to be grabbed in other helper sub-routines), then its
probably a bug - why not just fix it?

-Kevin

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