From: Waiman Long <longman@redhat.com>
To: Will Deacon <will.deacon@arm.com>, linux-kernel@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org, peterz@infradead.org,
mingo@kernel.org, boqun.feng@gmail.com,
paulmck@linux.vnet.ibm.com
Subject: Re: [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath
Date: Thu, 26 Apr 2018 16:16:30 -0400 [thread overview]
Message-ID: <1adce90b-7627-ed71-fd34-bb33388790d5@redhat.com> (raw)
In-Reply-To: <1524738868-31318-6-git-send-email-will.deacon@arm.com>
On 04/26/2018 06:34 AM, Will Deacon wrote:
> The qspinlock locking slowpath utilises a "pending" bit as a simple form
> of an embedded test-and-set lock that can avoid the overhead of explicit
> queuing in cases where the lock is held but uncontended. This bit is
> managed using a cmpxchg loop which tries to transition the uncontended
> lock word from (0,0,0) -> (0,0,1) or (0,0,1) -> (0,1,1).
>
> Unfortunately, the cmpxchg loop is unbounded and lockers can be starved
> indefinitely if the lock word is seen to oscillate between unlocked
> (0,0,0) and locked (0,0,1). This could happen if concurrent lockers are
> able to take the lock in the cmpxchg loop without queuing and pass it
> around amongst themselves.
>
> This patch fixes the problem by unconditionally setting _Q_PENDING_VAL
> using atomic_fetch_or, and then inspecting the old value to see whether
> we need to spin on the current lock owner, or whether we now effectively
> hold the lock. The tricky scenario is when concurrent lockers end up
> queuing on the lock and the lock becomes available, causing us to see
> a lockword of (n,0,0). With pending now set, simply queuing could lead
> to deadlock as the head of the queue may not have observed the pending
> flag being cleared. Conversely, if the head of the queue did observe
> pending being cleared, then it could transition the lock from (n,0,0) ->
> (0,0,1) meaning that any attempt to "undo" our setting of the pending
> bit could race with a concurrent locker trying to set it.
>
> We handle this race by preserving the pending bit when taking the lock
> after reaching the head of the queue and leaving the tail entry intact
> if we saw pending set, because we know that the tail is going to be
> updated shortly.
>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> ---
> kernel/locking/qspinlock.c | 102 ++++++++++++++++++++----------------
> kernel/locking/qspinlock_paravirt.h | 5 --
> 2 files changed, 58 insertions(+), 49 deletions(-)
>
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index a0f7976348f8..ad94b7def005 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -128,6 +128,17 @@ static inline __pure struct mcs_spinlock *decode_tail(u32 tail)
>
> #if _Q_PENDING_BITS == 8
> /**
> + * clear_pending - clear the pending bit.
> + * @lock: Pointer to queued spinlock structure
> + *
> + * *,1,* -> *,0,*
> + */
> +static __always_inline void clear_pending(struct qspinlock *lock)
> +{
> + WRITE_ONCE(lock->pending, 0);
> +}
> +
> +/**
> * clear_pending_set_locked - take ownership and clear the pending bit.
> * @lock: Pointer to queued spinlock structure
> *
> @@ -163,6 +174,17 @@ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
> #else /* _Q_PENDING_BITS == 8 */
>
> /**
> + * clear_pending - clear the pending bit.
> + * @lock: Pointer to queued spinlock structure
> + *
> + * *,1,* -> *,0,*
> + */
> +static __always_inline void clear_pending(struct qspinlock *lock)
> +{
> + atomic_andnot(_Q_PENDING_VAL, &lock->val);
> +}
> +
> +/**
> * clear_pending_set_locked - take ownership and clear the pending bit.
> * @lock: Pointer to queued spinlock structure
> *
> @@ -266,7 +288,7 @@ static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock,
> void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> {
> struct mcs_spinlock *prev, *next, *node;
> - u32 new, old, tail;
> + u32 old, tail;
> int idx;
>
> BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
> @@ -290,58 +312,50 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> }
>
> /*
> + * If we observe any contention; queue.
> + */
> + if (val & ~_Q_LOCKED_MASK)
> + goto queue;
> +
> + /*
> * trylock || pending
> *
> * 0,0,0 -> 0,0,1 ; trylock
> * 0,0,1 -> 0,1,1 ; pending
> */
> - for (;;) {
> + val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
> + if (!(val & ~_Q_LOCKED_MASK)) {
> /*
> - * If we observe any contention; queue.
> + * we're pending, wait for the owner to go away.
> + *
> + * *,1,1 -> *,1,0
> + *
> + * this wait loop must be a load-acquire such that we match the
> + * store-release that clears the locked bit and create lock
> + * sequentiality; this is because not all
> + * clear_pending_set_locked() implementations imply full
> + * barriers.
> */
> - if (val & ~_Q_LOCKED_MASK)
> - goto queue;
> -
> - new = _Q_LOCKED_VAL;
> - if (val == new)
> - new |= _Q_PENDING_VAL;
> + if (val & _Q_LOCKED_MASK) {
> + smp_cond_load_acquire(&lock->val.counter,
> + !(VAL & _Q_LOCKED_MASK));
> + }
>
> /*
> - * Acquire semantic is required here as the function may
> - * return immediately if the lock was free.
> + * take ownership and clear the pending bit.
> + *
> + * *,1,0 -> *,0,1
> */
> - old = atomic_cmpxchg_acquire(&lock->val, val, new);
> - if (old == val)
> - break;
> -
> - val = old;
> - }
> -
> - /*
> - * we won the trylock
> - */
> - if (new == _Q_LOCKED_VAL)
> + clear_pending_set_locked(lock);
> return;
> + }
>
> /*
> - * we're pending, wait for the owner to go away.
> - *
> - * *,1,1 -> *,1,0
> - *
> - * this wait loop must be a load-acquire such that we match the
> - * store-release that clears the locked bit and create lock
> - * sequentiality; this is because not all clear_pending_set_locked()
> - * implementations imply full barriers.
> - */
> - smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_MASK));
> -
> - /*
> - * take ownership and clear the pending bit.
> - *
> - * *,1,0 -> *,0,1
> + * If pending was clear but there are waiters in the queue, then
> + * we need to undo our setting of pending before we queue ourselves.
> */
> - clear_pending_set_locked(lock);
> - return;
> + if (!(val & _Q_PENDING_MASK))
> + clear_pending(lock);
>
> /*
> * End of pending bit optimistic spinning and beginning of MCS
> @@ -445,15 +459,15 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> * claim the lock:
> *
> * n,0,0 -> 0,0,1 : lock, uncontended
> - * *,0,0 -> *,0,1 : lock, contended
> + * *,*,0 -> *,*,1 : lock, contended
> *
> - * If the queue head is the only one in the queue (lock value == tail),
> - * clear the tail code and grab the lock. Otherwise, we only need
> - * to grab the lock.
> + * If the queue head is the only one in the queue (lock value == tail)
> + * and nobody is pending, clear the tail code and grab the lock.
> + * Otherwise, we only need to grab the lock.
> */
> for (;;) {
> /* In the PV case we might already have _Q_LOCKED_VAL set */
> - if ((val & _Q_TAIL_MASK) != tail) {
> + if ((val & _Q_TAIL_MASK) != tail || (val & _Q_PENDING_MASK)) {
> set_locked(lock);
> break;
> }
> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> index 2711940429f5..2dbad2f25480 100644
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -118,11 +118,6 @@ static __always_inline void set_pending(struct qspinlock *lock)
> WRITE_ONCE(lock->pending, 1);
> }
>
> -static __always_inline void clear_pending(struct qspinlock *lock)
> -{
> - WRITE_ONCE(lock->pending, 0);
> -}
> -
> /*
> * The pending bit check in pv_queued_spin_steal_lock() isn't a memory
> * barrier. Therefore, an atomic cmpxchg_acquire() is used to acquire the
There is another clear_pending() function after the "#else /*
_Q_PENDING_BITS == 8 */" line that need to be removed as well.
-Longman
next prev parent reply other threads:[~2018-04-26 20:16 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-26 10:34 [PATCH v3 00/14] kernel/locking: qspinlock improvements Will Deacon
2018-04-26 10:34 ` [PATCH v3 01/14] barriers: Introduce smp_cond_load_relaxed and atomic_cond_read_relaxed Will Deacon
2018-04-27 9:36 ` [tip:locking/core] locking/barriers: Introduce smp_cond_load_relaxed() and atomic_cond_read_relaxed() tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 02/14] locking/qspinlock: Merge struct __qspinlock into struct qspinlock Will Deacon
2018-04-27 9:37 ` [tip:locking/core] locking/qspinlock: Merge 'struct __qspinlock' into 'struct qspinlock' tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 03/14] locking/qspinlock: Bound spinning on pending->locked transition in slowpath Will Deacon
2018-04-27 9:37 ` [tip:locking/core] " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 04/14] locking/qspinlock/x86: Increase _Q_PENDING_LOOPS upper bound Will Deacon
2018-04-27 9:38 ` [tip:locking/core] " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath Will Deacon
2018-04-26 15:53 ` Peter Zijlstra
2018-04-26 16:55 ` Will Deacon
2018-04-28 12:45 ` Peter Zijlstra
2018-04-30 8:53 ` Will Deacon
2018-04-26 20:16 ` Waiman Long [this message]
2018-04-27 10:16 ` Will Deacon
2018-04-27 11:01 ` [tip:locking/core] locking/qspinlock: Remove duplicate clear_pending() function from PV code tip-bot for Will Deacon
2018-04-27 13:09 ` [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath Waiman Long
2018-04-27 9:39 ` [tip:locking/core] locking/qspinlock: Remove unbounded cmpxchg() " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 06/14] locking/qspinlock: Kill cmpxchg loop when claiming lock from head of queue Will Deacon
2018-04-27 9:39 ` [tip:locking/core] locking/qspinlock: Kill cmpxchg() " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 07/14] locking/qspinlock: Use atomic_cond_read_acquire Will Deacon
2018-04-27 9:40 ` [tip:locking/core] locking/qspinlock: Use atomic_cond_read_acquire() tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 08/14] locking/mcs: Use smp_cond_load_acquire() in mcs spin loop Will Deacon
2018-04-27 9:40 ` [tip:locking/core] locking/mcs: Use smp_cond_load_acquire() in MCS " tip-bot for Jason Low
2018-04-26 10:34 ` [PATCH v3 09/14] locking/qspinlock: Use smp_cond_load_relaxed to wait for next node Will Deacon
2018-04-27 9:41 ` [tip:locking/core] locking/qspinlock: Use smp_cond_load_relaxed() " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 10/14] locking/qspinlock: Make queued_spin_unlock use smp_store_release Will Deacon
2018-04-27 9:42 ` [tip:locking/core] locking/qspinlock: Use smp_store_release() in queued_spin_unlock() tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 11/14] locking/qspinlock: Elide back-to-back RELEASE operations with smp_wmb() Will Deacon
2018-04-27 9:42 ` [tip:locking/core] " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 12/14] locking/qspinlock: Use try_cmpxchg instead of cmpxchg when locking Will Deacon
2018-04-27 9:43 ` [tip:locking/core] locking/qspinlock: Use try_cmpxchg() instead of cmpxchg() " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 13/14] locking/qspinlock: Add stat tracking for pending vs slowpath Will Deacon
2018-04-27 9:43 ` [tip:locking/core] locking/qspinlock: Add stat tracking for pending vs. slowpath tip-bot for Waiman Long
2018-04-26 10:34 ` [PATCH v3 14/14] MAINTAINERS: Add myself as a co-maintainer for LOCKING PRIMITIVES Will Deacon
2018-04-26 15:55 ` Peter Zijlstra
2018-04-27 9:44 ` [tip:locking/core] MAINTAINERS: Add myself as a co-maintainer for the locking subsystem tip-bot for Will Deacon
2018-04-26 15:54 ` [PATCH v3 00/14] kernel/locking: qspinlock improvements Peter Zijlstra
2018-04-27 9:33 ` Ingo Molnar
2018-04-26 20:18 ` Waiman Long
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