Hi all, Thanks for reviewing.
Do you have any other concerns about this small change? Thanks Baoquan On 12/17/14 at 04:16pm, Waiman Long wrote: > On 12/16/2014 10:36 AM, Baoquan He wrote: > >On 12/16/14 at 10:01am, Peter Zijlstra wrote: > >>On Tue, Dec 16, 2014 at 02:00:40PM +0800, Baoquan He wrote: > >>>In queue_read_lock_slowpath, when writer count becomes 0, we need > >>>increment the read count and get the lock. Then need call > >>>rspin_until_writer_unlock to check again if an incoming writer > >>>steals the lock in the gap. But in rspin_until_writer_unlock > >>>it only checks the writer count, namely low 8 bit of lock->cnts, > >>>no need to subtract the reader count unit specifically. So remove > >>>that subtraction to make it clearer, rspin_until_writer_unlock > >>>just takes the actual lock->cnts as the 2nd argument. > >>> > >>>And also change the code comment in queue_write_lock_slowpath to > >>>make it more exact and explicit. > >>> > >>>Signed-off-by: Baoquan He<[email protected]> > >>>--- > >>> kernel/locking/qrwlock.c | 8 ++++---- > >>> 1 file changed, 4 insertions(+), 4 deletions(-) > >>> > >>>diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c > >>>index f956ede..ae66c10 100644 > >>>--- a/kernel/locking/qrwlock.c > >>>+++ b/kernel/locking/qrwlock.c > >>>@@ -76,7 +76,7 @@ void queue_read_lock_slowpath(struct qrwlock *lock) > >>> while (atomic_read(&lock->cnts)& _QW_WMASK) > >>> cpu_relax_lowlatency(); > >>> > >>>- cnts = atomic_add_return(_QR_BIAS,&lock->cnts) - _QR_BIAS; > >>>+ cnts = atomic_add_return(_QR_BIAS,&lock->cnts); > >>> rspin_until_writer_unlock(lock, cnts); > >>Did you actually look at the ASM generated? I suspect your change makes > >>it bigger. > > > >It does make it bigger. But it doesn't matter. Because in > >rspin_until_writer_unlock it only compqre (cnts& _QW_WMASK) > >with _QW_LOCKED. So using incremented reader count doesn't impact > >the result. Anyway it will get the actual lock->cnts in > >rspin_until_writer_unlock in next loop. I can't see why we need > >subtract that reader count increment specifically. > > > >When I read this code, thought there's some special usage. Finally I > >realized it doesn't have special usage, and doesn't have to do that. > > The "- _QR_BIAS" expression was added to simulate xadd() which is > present in x86, but not in some other architectures. There is no > equivalent functionality in the set of atomic helper functions. > Anyway, I have no objection to the change as it is in the slowpath. > > Acked-by: Waiman Long <[email protected]> > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to [email protected] > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
