Re: [PATCH v5 3/3] locking/qrwlock: Don't contend with readers when setting _QW_WAITING

Will Deacon
2015-06-22T16:21:35Z

Hi Waiman,

On Fri, Jun 19, 2015 at 04:50:02PM +0100, Waiman Long wrote:
> The current cmpxchg() loop in setting the _QW_WAITING flag for writers
> in queue_write_lock_slowpath() will contend with incoming readers
> causing possibly extra cmpxchg() operations that are wasteful. This
> patch changes the code to do a byte cmpxchg() to eliminate contention
> with new readers.

[...]

> diff --git a/arch/x86/include/asm/qrwlock.h b/arch/x86/include/asm/qrwlock.h
> index a8810bf..5678b0a 100644
> --- a/arch/x86/include/asm/qrwlock.h
> +++ b/arch/x86/include/asm/qrwlock.h
> @@ -7,8 +7,7 @@
> #define queued_write_unlock queued_write_unlock
> static inline void queued_write_unlock(struct qrwlock *lock)
> {
> - barrier();
> - ACCESS_ONCE(*(u8 *)&lock->cnts) = 0;
> + smp_store_release(&lock->wmode, 0);
> }
> #endif

I reckon you could actually use this in the asm-generic header and remove
the x86 arch version altogether. Most architectures support single-copy
atomic byte access and those that don't (alpha?) can just not use qrwlock
(or override write_unlock with atomic_sub).

I already have a patch making this change, so I'm happy either way.

> diff --git a/include/asm-generic/qrwlock_types.h b/include/asm-generic/qrwlock_types.h
> index 4d76f24..d614cde 100644
> --- a/include/asm-generic/qrwlock_types.h
> +++ b/include/asm-generic/qrwlock_types.h
> @@ -3,13 +3,29 @@
>
> #include <linux/types.h>
> #include <asm/spinlock_types.h>
> +#include <asm/byteorder.h>
>
> /*
> * The queue read/write lock data structure
> + *
> + * The 32-bit count is divided into an 8-bit writer mode byte
> + * (least significant byte) and a 24-bit reader count.
> + *
> */
>
> typedef struct qrwlock {
> - atomic_t cnts;
> + union {
> + atomic_t cnts;
> + struct {
> +#ifdef __LITTLE_ENDIAN
> + u8 wmode; /* Writer mode */
> + u8 rcnt[3]; /* Reader count */
> +#else
> + u8 rcnt[3]; /* Reader count */
> + u8 wmode; /* Writer mode */
> +#endif
> + };
> + };
> arch_spinlock_t lock;
> } arch_rwlock_t;
>
> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> index 26ca0ca..a7ac2c5 100644
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -108,10 +108,8 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
> * or wait for a previous writer to go away.
> */
> for (;;) {
> - cnts = atomic_read(&lock->cnts);
> - if (!(cnts & _QW_WMASK) &&
> - (atomic_cmpxchg(&lock->cnts, cnts,
> - cnts | _QW_WAITING) == cnts))
> + if (!READ_ONCE(lock->wmode) &&
> + (cmpxchg(&lock->wmode, 0, _QW_WAITING) == 0))
> break;

Reviewed-by: Will Deacon <will.deacon@arm.com>

Will