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SubjectRe: [RFC PATCH v1 02/25] printk-rb: add prb locking functions
On Wed 2019-02-13 22:39:20, John Ogness wrote:
> On 2019-02-13, Petr Mladek <pmladek@suse.com> wrote:
> >> +/*
> >> + * prb_unlock: Perform a processor-reentrant spin unlock.
> >> + * @cpu_lock: A pointer to the lock object.
> >> + * @cpu_store: A "flags" object storing lock status information.
> >> + *
> >> + * Release the lock. The calling processor must be the owner of the lock.
> >> + *
> >> + * It is safe to call this function from any context and state.
> >> + */
> >> +void prb_unlock(struct prb_cpulock *cpu_lock, unsigned int cpu_store)
> >> +{
> >> + unsigned long *flags;
> >> + unsigned int cpu;
> >> +
> >> + cpu = atomic_read(&cpu_lock->owner);
> >> + atomic_set_release(&cpu_lock->owner, cpu_store);
> >> +
> >> + if (cpu_store == -1) {
> >> + flags = per_cpu_ptr(cpu_lock->irqflags, cpu);
> >> + local_irq_restore(*flags);
> >> + }
> >
> > cpu_store looks like an implementation detail. The caller
> > needs to remember it to handle the nesting properly.
>
> It's really no different than "flags" in irqsave/irqrestore.
>
> > We could achieve the same with a recursion counter hidden
> > in struct prb_lock.
>
> The only way I see how that could be implemented is if the cmpxchg
> encoded the cpu owner and counter into a single integer. (Upper half as
> counter, lower half as cpu owner.) Both fields would need to be updated
> with a single cmpxchg. The critical cmpxchg being the one where the CPU
> becomes unlocked (counter goes from 1 to 0 and cpu owner goes from N to
> -1).

The atomic operations are tricky. I feel other lost in them.
Well, I still think that it might easier to detect nesting
on the same CPU, see below.

Also there is no need to store irq flags in per-CPU variable.
Only the first owner of the lock need to store the flags. The others
are spinning or nested.

struct prb_cpulock {
atomic_t owner;
unsigned int flags;
int nesting; /* intialized to 0 */
};

void prb_lock(struct prb_cpulock *cpu_lock)
{
unsigned int flags;
int cpu;

/*
* The next condition might be valid only when
* we are nested on the same CPU. It means
* the IRQs are already disabled and no
* memory barrier is needed.
*/
if (cpu_lock->owner == smp_processor_id()) {
cpu_lock->nested++;
return;
}

/* Not nested. Take the lock */
local_irq_save(flags);
cpu = smp_processor_id();

for (;;) {
if (atomic_try_cmpxchg_acquire(&cpu_lock->owner,
-1, cpu)) {
cpu_lock->flags = flags;
break;
}

cpu_relax();
}
}

void prb_unlock(struct prb_cpulock *cpu_lock)
{
unsigned int flags;

if (cpu_lock->nested)
cpu_lock->nested--;
return;
}

/* We must be the first lock owner */
flags = cpu_lock->flags;
atomic_set_release(&cpu_lock->owner, -1);
local_irq_restore(flags);
}

Or do I miss anything?

Best Regards,
Petr

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