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SubjectRe: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles
Hi Oleg,

On 02/18/2015 04:59 PM, Oleg Nesterov wrote:
> Let's look at sem_lock(). I never looked at this code before, I can be
> easily wrong. Manfred will correct me. But at first glance we can write
> the oversimplified pseudo-code:
>
> spinlock_t local, global;
>
> bool my_lock(bool try_local)
> {
> if (try_local) {
> spin_lock(&local);
> if (!spin_is_locked(&global))
> return true;
> spin_unlock(&local);
> }
>
> spin_lock(&global);
> spin_unlock_wait(&local);
> return false;
> }
>
> void my_unlock(bool drop_local)
> {
> if (drop_local)
> spin_unlock(&local);
> else
> spin_unlock(&global);
> }
>
> it assumes that the "local" lock is cheaper than "global", the usage is
>
> bool xxx = my_lock(condition);
> /* CRITICAL SECTION */
> my_unlock(xxx);
>
> Now. Unless I missed something, my_lock() does NOT need a barrier BEFORE
> spin_unlock_wait() (or spin_is_locked()). Either my_lock(true) should see
> spin_is_locked(global) == T, or my_lock(false)->spin_unlock_wait() should
> see that "local" is locked and wait.
I would agree:
There is no need for a barrier. spin_unlock_read() is just a read, the
barriers are from spin_lock() and spin_unlock().

The barrier exist to protect something like a "force_global" flag
(complex_count)

> spinlock_t local, global;
> bool force_global;
> bool my_lock(bool try_local)
> {
> if (try_local) {
> spin_lock(&local);
> if (!spin_is_locked(&global)) {
> if (!force_global) {
> return true;
> }
> }
> spin_unlock(&local);
>
>
> spin_lock(&global);
> spin_unlock_wait(&local);
> return false;
> }
>
> void my_unlock(bool drop_local)
> {
> if (drop_local)
> spin_unlock(&local);
> else
> spin_unlock(&global);
> }
> }

force_global can only be set by the owner of &global.

> Another question is do we need a barrier AFTER spin_unlock_wait(). I do not
> know what ipc/sem.c actually needs, but in general (I think) this does need
> mb(). Otherwise my_lock / my_unlock itself does not have the proper acq/rel
> semantics. For example, my_lock(false) can miss the changes which were done
> under my_lock(true).
How could that happen?
I thought that
thread A:
protected_var = 1234;
spin_unlock(&lock_a)

thread B:
spin_lock(&lock_b)
if (protected_var)

is safe. i.e, there is no need that acquire and releases is done on the same pointer.

--
Manfred




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