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Andrew Morton wrote: > 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); 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. Send instant messages to your online friends http://au.messenger.yahoo.com - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||
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