On Tue, Jan 14, 2014 at 11:44:06PM -0500, Waiman Long wrote:
> This patch modifies the queue_write_unlock() function to use the new
> smp_store_release() function (currently in tip). It also removes the
> temporary implementation of smp_load_acquire() and smp_store_release()
> function in qrwlock.c.
>
> This patch will use atomic subtraction instead if the writer field is
> not atomic.
>
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
> ---
> include/asm-generic/qrwlock.h | 10 ++++++----
> kernel/locking/qrwlock.c | 34 ----------------------------------
> 2 files changed, 6 insertions(+), 38 deletions(-)
>
> diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
> index 5abb6ca..68f488b 100644
> --- a/include/asm-generic/qrwlock.h
> +++ b/include/asm-generic/qrwlock.h
> @@ -181,11 +181,13 @@ static inline void queue_read_unlock(struct qrwlock *lock)
> static inline void queue_write_unlock(struct qrwlock *lock)
> {
> /*
> - * Make sure that none of the critical section will be leaked out.
> + * If the writer field is atomic, it can be cleared directly.
> + * Otherwise, an atomic subtraction will be used to clear it.
> */
> - smp_mb__before_clear_bit();
> - ACCESS_ONCE(lock->cnts.writer) = 0;
> - smp_mb__after_clear_bit();
> + if (__native_word(lock->cnts.writer))
> + smp_store_release(&lock->cnts.writer, 0);
> + else
> + atomic_sub(_QW_LOCKED, &lock->cnts.rwa);
> }
If we're a writer, read-count must be zero. The only way for that not to
be zero is a concurrent read-(try)lock. If you move all the
read-(try)locks over to cmpxchg() you can avoid this afaict:
static inline void queue_read_trylock(struct qrwlock *lock)
{
union qrwcnts cnts
cnts = ACCESS_ONCE(lock->cnts);
if (!cnts.writer) {
if (cmpxchg(&lock->cnts.rwc, cnts.rwc, cnts.rwc + _QR_BIAS) == cnts.rwc)
return 1;
}
return 0;
}
static inline void queue_read_lock(struct qrwlock *lock)
{
if (!queue_read_trylock(lock))
queue_read_lock_slowpath(); // XXX do not assume extra _QR_BIAS
}
At which point you have the guarantee that read-count == 0, and you can
write:
static inline void queue_write_unlock(struct qrwlock *lock)
{
smp_store_release(&lock->cnts.rwc, 0);
}
No?
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