Re: [PATCH] Fix x86_64 _spin_lock_irqsave()

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Andrew Morton wrote:
On 24 Aug 2006 08:45:11 +0200
Andi Kleen <[email protected]> wrote:


Edward Falk <[email protected]> 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);

Yes, or it may be taken inside a section that needs interrupts off for
correctness (eg. if it is holding an irq safe lock). And in the current
implementation I don't think the plain _irq variants reenable interrupts
because that would require reading the register.

Would it be sufficient to just do pair-wise rendezvous, where the
initiating CPU is in a known good state? For TSC sync it might be...

--
SUSE Labs, Novell Inc.
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