Messages in this thread | | | Date | Tue, 16 Dec 2014 23:36:50 +0800 | From | Baoquan He <> | Subject | Re: [PATCH] locking/rwlocks: clean up of qrwlock |
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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 <bhe@redhat.com> > > --- > > 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.
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