Messages in this thread | | | Date | Tue, 8 Jun 2021 16:49:48 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH next v2 2/2] printk: fix cpu lock ordering |
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On Tue 2021-06-08 16:18:51, John Ogness wrote: > On 2021-06-08, Petr Mladek <pmladek@suse.com> wrote: > > The change makes perfect sense and the code looks correct. > > But I am not sure about the description of the memory barriers. > > OK. > > >> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > >> index f94babb38493..8c870581cfb4 100644 > >> --- a/kernel/printk/printk.c > >> +++ b/kernel/printk/printk.c > >> @@ -3560,10 +3560,29 @@ void printk_cpu_lock_irqsave(bool *lock_flag, unsigned long *irq_flags) > >> > >> cpu = smp_processor_id(); > >> > >> - old = atomic_cmpxchg(&printk_cpulock_owner, -1, cpu); > >> + /* > >> + * Guarantee loads and stores from the previous lock owner are > >> + * visible to this CPU once it is the lock owner. This pairs > >> + * with cpu_unlock:B. > > > > These things are not easy to describe. It took me quite some time to > > understand the above description. And think that it does not say > > the full storry. > > > > IMHO, the lock should work the way that: > > > > + The new owner see all writes done or seen by the previous owner(s). > > + The previous owner(s) never see writes done by the new owner. > > You are right. I can describe those independently. > > > Honestly, I am not sure if we could describe the barriers correctly > > and effectively at the same time. > > For v3 I would describe the 2 cases separately. For lock/acquire: > > /* > * Guarantee loads and stores from this CPU when it is the lock owner > * are _not_ visible to the previous lock owner. This pairs with > * cpu_unlock:B.
Sounds better to me.
* > * Memory barrier involvement: > * > * If cpu_lock:A reads from cpu_unlock:B, then cpu_unlock:A can never > * read from cpu_lock:B. > * > * Relies on: > * > * RELEASE from cpu_unlock:A to cpu_unlock:B > * matching > * ACQUIRE from cpu_lock:A to cpu_lock:B > */
I can't check this so I believe you ;-)
> And for unlock/release: > > /* > * Guarantee loads and stores from this CPU when it was the > * lock owner are visible to the next lock owner. This pairs > * with cpu_lock:A.
Sounds good.
* > * Memory barrier involvement: > * > * If cpu_lock:A reads from cpu_unlock:B, then cpu_lock:B > * reads from cpu_unlock:A. > * > * Relies on: > * > * RELEASE from cpu_unlock:A to cpu_unlock:B > * matching > * ACQUIRE from cpu_lock:A to cpu_lock:B > */
Same as for acquire ;-)
> I know you are not a fan of these drawn out memory barrier comments. But > it really simplifies verification and translation to litmus > tests. Without such comments, I would be lost looking back at > printk_ringbuffer.c. > > If the previous dump_stack() cpu lock implementation had such comments, > we would know if the missing memory barriers were by design.
Sure. I am fine with the comments as long as there is also some human readable description of the barrier intention.
Best Regards, Petr
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