similar to: [PATCH V2] x86 spinlock: Fix memory corruption on completing completions

Paravirt spinlock clears slowpath flag after doing unlock. As explained by Linus currently it does: prev = *lock; add_smp(&lock->tickets.head, TICKET_LOCK_INC); /* add_smp() is a full mb() */ if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) __ticket_unlock_slowpath(lock, prev); which

Paravirt spinlock clears slowpath flag after doing unlock. As explained by Linus currently it does: prev = *lock; add_smp(&lock->tickets.head, TICKET_LOCK_INC); /* add_smp() is a full mb() */ if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) __ticket_unlock_slowpath(lock, prev); which

Paravirt spinlock clears slowpath flag after doing unlock. As explained by Linus currently it does: prev = *lock; add_smp(&lock->tickets.head, TICKET_LOCK_INC); /* add_smp() is a full mb() */ if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) __ticket_unlock_slowpath(lock, prev); which

Paravirt spinlock clears slowpath flag after doing unlock. As explained by Linus currently it does: prev = *lock; add_smp(&lock->tickets.head, TICKET_LOCK_INC); /* add_smp() is a full mb() */ if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) __ticket_unlock_slowpath(lock, prev); which

Paravirt spinlock clears slowpath flag after doing unlock. As explained by Linus currently it does: prev = *lock; add_smp(&lock->tickets.head, TICKET_LOCK_INC); /* add_smp() is a full mb() */ if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) __ticket_unlock_slowpath(lock, prev); which is

Paravirt spinlock clears slowpath flag after doing unlock. As explained by Linus currently it does: prev = *lock; add_smp(&lock->tickets.head, TICKET_LOCK_INC); /* add_smp() is a full mb() */ if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) __ticket_unlock_slowpath(lock, prev); which is

Paravirt spinlock clears slowpath flag after doing unlock. As explained by Linus currently it does: prev = *lock; add_smp(&lock->tickets.head, TICKET_LOCK_INC); /* add_smp() is a full mb() */ if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) __ticket_unlock_slowpath(lock, prev); which is

Paravirt spinlock clears slowpath flag after doing unlock. As explained by Linus currently it does: prev = *lock; add_smp(&lock->tickets.head, TICKET_LOCK_INC); /* add_smp() is a full mb() */ if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) __ticket_unlock_slowpath(lock, prev); which is

On 02/09, Raghavendra K T wrote: > > +static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock) > +{ > + arch_spinlock_t old, new; > + __ticket_t diff; > + > + old.tickets = READ_ONCE(lock->tickets); > + diff = (old.tickets.tail & ~TICKET_SLOWPATH_FLAG) - old.tickets.head; > + > + /* try to clear slowpath flag when there are no contenders */

On 02/09, Raghavendra K T wrote: > > +static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock) > +{ > + arch_spinlock_t old, new; > + __ticket_t diff; > + > + old.tickets = READ_ONCE(lock->tickets); > + diff = (old.tickets.tail & ~TICKET_SLOWPATH_FLAG) - old.tickets.head; > + > + /* try to clear slowpath flag when there are no contenders */

On 02/09/2015 02:44 AM, Jeremy Fitzhardinge wrote: > On 02/06/2015 06:49 AM, Raghavendra K T wrote: [...] > >> Linus suggested that we should not do any writes to lock after unlock(), >> and we can move slowpath clearing to fastpath lock. > > Yep, that seems like a sound approach. Current approach seem to be working now. (though we could not avoid read). Related question:

On 02/09/2015 02:44 AM, Jeremy Fitzhardinge wrote: > On 02/06/2015 06:49 AM, Raghavendra K T wrote: [...] > >> Linus suggested that we should not do any writes to lock after unlock(), >> and we can move slowpath clearing to fastpath lock. > > Yep, that seems like a sound approach. Current approach seem to be working now. (though we could not avoid read). Related question:

On 02/06/2015 06:49 AM, Raghavendra K T wrote: > Paravirt spinlock clears slowpath flag after doing unlock. > As explained by Linus currently it does: > prev = *lock; > add_smp(&lock->tickets.head, TICKET_LOCK_INC); > > /* add_smp() is a full mb() */ > > if (unlikely(lock->tickets.tail &

On 02/06/2015 06:49 AM, Raghavendra K T wrote: > Paravirt spinlock clears slowpath flag after doing unlock. > As explained by Linus currently it does: > prev = *lock; > add_smp(&lock->tickets.head, TICKET_LOCK_INC); > > /* add_smp() is a full mb() */ > > if (unlikely(lock->tickets.tail &

In virtualized environment there are mainly three problems related to spinlocks that affect performance. 1. LHP (lock holder preemption) 2. Lock Waiter Preemption (LWP) 3. Starvation/fairness Though ticketlocks solve the fairness problem, it worsens LWP, LHP problems. pv-ticketlocks tried to address this. But we can further improve at the cost of relaxed fairness. In this patch, we form a batch

In virtualized environment there are mainly three problems related to spinlocks that affect performance. 1. LHP (lock holder preemption) 2. Lock Waiter Preemption (LWP) 3. Starvation/fairness Though ticketlocks solve the fairness problem, it worsens LWP, LHP problems. pv-ticketlocks tried to address this. But we can further improve at the cost of relaxed fairness. In this patch, we form a batch

* Raghavendra K T <raghavendra.kt at linux.vnet.ibm.com> [2015-02-15 11:25:44]: Resending the V5 with smp_mb__after_atomic() change without bumping up revision ---8<--- >From 0b9ecde30e3bf5b5b24009fd2ac5fc7ac4b81158 Mon Sep 17 00:00:00 2001 From: Raghavendra K T <raghavendra.kt at linux.vnet.ibm.com> Date: Fri, 6 Feb 2015 16:44:11 +0530 Subject: [PATCH RESEND V5] x86 spinlock:

* Raghavendra K T <raghavendra.kt at linux.vnet.ibm.com> [2015-02-15 11:25:44]: Resending the V5 with smp_mb__after_atomic() change without bumping up revision ---8<--- >From 0b9ecde30e3bf5b5b24009fd2ac5fc7ac4b81158 Mon Sep 17 00:00:00 2001 From: Raghavendra K T <raghavendra.kt at linux.vnet.ibm.com> Date: Fri, 6 Feb 2015 16:44:11 +0530 Subject: [PATCH RESEND V5] x86 spinlock:

On 02/10, Raghavendra K T wrote: > > On 02/10/2015 06:23 AM, Linus Torvalds wrote: > >> add_smp(&lock->tickets.head, TICKET_LOCK_INC); >> if (READ_ONCE(lock->tickets.tail) & TICKET_SLOWPATH_FLAG) .. >> >> into something like >> >> val = xadd((&lock->ticket.head_tail, TICKET_LOCK_INC << TICKET_SHIFT);

On 02/10, Raghavendra K T wrote: > > On 02/10/2015 06:23 AM, Linus Torvalds wrote: > >> add_smp(&lock->tickets.head, TICKET_LOCK_INC); >> if (READ_ONCE(lock->tickets.tail) & TICKET_SLOWPATH_FLAG) .. >> >> into something like >> >> val = xadd((&lock->ticket.head_tail, TICKET_LOCK_INC << TICKET_SHIFT);