Re: [PATCH] Fix x86_64 _spin_lock_irqsave()

[Andrew Morton <akpm@osdl.org>]

On 24 Aug 2006 08:45:11 +0200
Andi Kleen <ak@suse.de> wrote:

> Edward Falk <efalk@google.com> writes:
> 
> > Add spin_lock_string_flags and _raw_spin_lock_flags() to
> > asm-x86_64/spinlock.h so that _spin_lock_irqsave() has the same
> > semantics on x86_64 as it does on i386 and does *not* have interrupts
> > disabled while it is waiting for the lock.
> 
> Did it fix anything for you?
> 

It's the rendezvous-via-IPI problem.  Suppose we want to capture all CPUs
in an IPI handler (TSC sync, for example).

- CPUa holds read_lock(&tasklist_lock)
- CPUb is spinning in write_lock_irq(&taslist_lock)
- CPUa enters its IPI handler and spins
- CPUb never takes the IPI and we're dead.

Re-enabling interrupts while we spin will prevent that.  But I suspect that
if we ever want to implement IPI rendezvous (and cannot use the
stop_machine_run() thing) then we might still have problems.  A valid
optimisation (which we use in some places) is:

	local_irq_save(flags);
	<stuff>
	write_lock(lock);