Re: probes, interrupt handling, etc

[William Cohen <wcohen at redhat dot com>]

Frank Ch. Eigler wrote:
> Hi -
>
>
> [moving to systemtap@sourceware]
>
> wcohen wrote:
>
>
> > [...] It appears that [kprobe_lock] is for accessing the data structure
> > that holds the information about which addresses have kprobe int3 set
> > and the appropriate handler for them.
>
>
> That would make sense, over the duration where actual shared
> data is being accessed.
>
>
> > It seems like this could be replaced with multiple reader/one writer 
> > locks to allow concurrency in handling the kprobes.
>
>
> I believe the only reader/writer locking primitives in the kernel
> involve semaphores (which can block/sleep), not spinlocks.  Unless
> I'm mistaken, this makes them unsuitable for use from within an
> interrupt context.

There are read/write spinlocks to improve concurrency. You can see them 
used in the ext2 file system and a number of device drivers.  The 
read/write spinlocks are defined in spinlock.h. For example:

#ifdef CONFIG_SMP
#define spin_lock_irqsave(lock, flags)	flags = _spin_lock_irqsave(lock)
#define read_lock_irqsave(lock, flags)	flags = _read_lock_irqsave(lock)
#define write_lock_irqsave(lock, flags)	flags = _write_lock_irqsave(lock)
#else
#define spin_lock_irqsave(lock, flags)	_spin_lock_irqsave(lock, flags)
#define read_lock_irqsave(lock, flags)	_read_lock_irqsave(lock, flags)
#define write_lock_irqsave(lock, flags)	_write_lock_irqsave(lock, flags)
#endif

> This brings up an interesting issue that relates to lifecycle
> management of the probes: specifically setting a good
> synchronization policy amongst the asynchronous probe functions
> and user-level supervisor startup/suspend/shutdown operations.
>
> I have a little state transition diagram that models a
> generalized form of the situation, which (for a small fee
> or taunt) I am willing to embarrass myself with by posting.

However, you are right there are still the asynchronous 
startup/suspend/shutdown operation issues.

I have to taut you to get you to post the state transition diagram? What 
was used to draw it? PowerPoint?

--Will