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SubjectRe: [PATCH 5/6] kernel/locking: Prevent slowpath writers getting held up by fastpath
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On 10/05/2017 09:56 AM, Peter Zijlstra wrote:
> On Thu, Oct 05, 2017 at 01:54:56PM +0100, Will Deacon wrote:
>> When a prospective writer takes the qrwlock locking slowpath due to the
>> lock being held, it attempts to cmpxchg the wmode field from 0 to
>> _QW_WAITING so that concurrent lockers also take the slowpath and queue
>> on the spinlock accordingly, allowing the lockers to drain.
>>
>> Unfortunately, this isn't fair, because a fastpath writer that comes in
>> after the lock is made available but before the _QW_WAITING flag is set
>> can effectively jump the queue. If there is a steady stream of prospective
>> writers, then the waiter will be held off indefinitely.
>>
>> This patch restores fairness by separating _QW_WAITING and _QW_LOCKED
>> into two bits in the wmode byte and having the waiter set _QW_WAITING
>> unconditionally. This then forces the slow-path for concurrent lockers,
>> but requires that a writer unlock operation performs an
>> atomic_sub_release instead of a store_release so that the waiting status
>> is preserved.
>> diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
>> index 02c0a768e6b0..8b7edef500e5 100644
>> --- a/include/asm-generic/qrwlock.h
>> +++ b/include/asm-generic/qrwlock.h
>> @@ -41,7 +41,7 @@
>> * +----+----+----+----+
>> */
>> #define _QW_WAITING 1 /* A writer is waiting */
>> -#define _QW_LOCKED 0xff /* A writer holds the lock */
>> +#define _QW_LOCKED 2 /* A writer holds the lock */
>> #define _QW_WMASK 0xff /* Writer mask */
>> #define _QR_SHIFT 8 /* Reader count shift */
>> #define _QR_BIAS (1U << _QR_SHIFT)
>> @@ -134,7 +134,7 @@ static inline void queued_read_unlock(struct qrwlock *lock)
>> */
>> static inline void queued_write_unlock(struct qrwlock *lock)
>> {
>> - smp_store_release(&lock->wmode, 0);
>> + (void)atomic_sub_return_release(_QW_LOCKED, &lock->cnts);
>> }
> That is a fairly painful hit on x86. Changes a regular store into an
> "LOCK XADD" +20 cycles right there.
>
> Can't we steal one of the reader bits for waiting?
>
> diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
> index 7d026bf27713..5524801a02a5 100644
> --- a/include/asm-generic/qrwlock.h
> +++ b/include/asm-generic/qrwlock.h
> @@ -40,10 +40,10 @@
> * | rd | wr |
> * +----+----+----+----+
> */
> -#define _QW_WAITING 1 /* A writer is waiting */
> -#define _QW_LOCKED 0xff /* A writer holds the lock */
> -#define _QW_WMASK 0xff /* Writer mask */
> -#define _QR_SHIFT 8 /* Reader count shift */
> +#define _QW_WAITING 0x100 /* A writer is waiting */
> +#define _QW_LOCKED 0x0ff /* A writer holds the lock */
> +#define _QW_WMASK 0x1ff /* Writer mask */
> +#define _QR_SHIFT 9 /* Reader count shift */
> #define _QR_BIAS (1U << _QR_SHIFT)
>
> /*
> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> index 2655f26ec882..5f75caea97e0 100644
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -54,7 +54,7 @@ struct __qrwlock {
> static __always_inline void
> rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
> {
> - while ((cnts & _QW_WMASK) == _QW_LOCKED) {
> + while ((cnts & _QW_LOCKED)) {
> cpu_relax();
> cnts = atomic_read_acquire(&lock->cnts);
> }
> @@ -120,21 +120,10 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
> (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
> goto unlock;
>
> - /*
> - * Set the waiting flag to notify readers that a writer is pending,
> - * or wait for a previous writer to go away.
> - */
> - for (;;) {
> - struct __qrwlock *l = (struct __qrwlock *)lock;
> -
> - if (!READ_ONCE(l->wmode) &&
> - (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0))
> - break;
> -
> - cpu_relax();
> - }
> + /* Set the waiting flag to notify readers that a writer is pending */
> + atomic_add(_QW_WAITING, &lock->cnts);
>
> - /* When no more readers, set the locked flag */
> + /* When no more readers or writers, set the locked flag */
> for (;;) {
> cnts = atomic_read(&lock->cnts);
> if ((cnts == _QW_WAITING) &&

I think this is the better of dealing with the fairness problem.

Acked-by: Waiman Long <longman@redhat.com>

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