Ping-Ke Shih <pkshih@xxxxxxxxxxx> writes: > On Tue, 2023-04-04 at 10:37 +0800, Ping-Ke Shih wrote: > >> On Mon, 2023-04-03 at 13:32 +0300, Kalle Valo wrote: >> > I would expect that there's polling if you are waiting something from >> > hardware, or maybe when implementing a spin lock, but not when waiting >> > for another kernel thread. This just doesn't feel right but I don't have >> > time to propose a good alternative either, sorry. >> > >> >> I have found a solution that uses an owner variable with a spin lock >> to determine which side to free completion object. Simply show two use >> cases below: >> >> Use case 1: (normal case; free completion object by work 1) >> work 1 work 2 >> wait >> (spin_lock) >> complete >> check & set owner --> owner = work1 >> (spin_unlock) >> wait ok >> (spin_lock) >> check & check owner --> free by work 1 >> (spin_unlock) >> free completion >> >> >> Use case 2: (timeout case; free completion object by work 2) >> work 1 work 2 >> wait >> wait timeout >> (spin_lock) >> check & set owner --> owner = work 2 >> (spin_unlock) >> (spin_lock) >> completion >> check & set owner --> free by work 2 >> (spin_unlock) >> free completion >> >> I will apply this by v5. >> > > We have a better idea that use kfree_rcu() to free completion, so no > need spin_lock(). Then, the use cases become below, and I have sent > this change by v6. > > Use case 1: (normal case) > work 1 work 2 > (rcu_assign_pointer(wait)) > wait > (rcu_read_lock) > wait = rcu_dereference(); > if (wait) --> wait != NULL > complete > (rcu_read_unlock) > wait ok > (rcu_assign_pointer(NULL)) > kfree_rcu(completion) > > > Use case 2: (timeout case) > work 1 work 2 > (rcu_assign_pointer(wait)) > wait > wait timeout > (rcu_assign_pointer(NULL)) > kfree_rcu(completion) > (rcu_read_lock) > wait = rcu_dereference(); > if (wait) --> wait == NULL > complete > (rcu_read_unlock) Thanks, I haven't had a chance to look at v6 yet (nor v5 for that matter) but at least based on this mail your idea looks much better than polling. -- https://patchwork.kernel.org/project/linux-wireless/list/ https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches
