On 07/30/2015 10:44 AM, Peter Zijlstra wrote:
On Fri, Jul 17, 2015 at 04:29:24PM -0700, Paul E. McKenney wrote:/* + * First try directly acquiring the root lock in order to reduce + * latency in the common case where expedited grace periods are + * rare. We check mutex_is_locked() to avoid pathological levels of + * memory contention on ->exp_funnel_mutex in the heavy-load case. + */ + rnp0 = rcu_get_root(rsp); + if (!mutex_is_locked(&rnp0->exp_funnel_mutex)) { + if (mutex_trylock(&rnp0->exp_funnel_mutex)) { + if (sync_exp_work_done(rsp, rnp0, NULL, + &rsp->expedited_workdone0, s)) + return NULL; + return rnp0; + } + }So our 'new' locking primitives do things like: static __always_inline int queued_spin_trylock(struct qspinlock *lock) { if (!atomic_read(&lock->val)&& (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) == 0)) return 1; return 0; } mutexes do not do this. Now I suppose the question is, does that extra read slow down the (common) uncontended case? (remember, we should optimize locks for the uncontended case, heavy lock contention should be fixed with better locking schemes, not lock implementations).
I suppose the extra read may slow down the uncontended case, but I am not sure by how much as I haven't run any test to quantify this. However, there are use cases where it is advantageous to do a read first, like when the lock cacheline is likely to be hot (in the slowpath, for example). So it depends on how the trylock is being used.
Cheers, Longman -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
