linux-kernel - Re: [BUG]locking/rwsem: only clean RWSEM_FLAG

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Date:   Thu, 11 Nov 2021 22:53:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Hillf Danton <hdanton@...a.com>,
        马振华 <mazhenhua@...omi.com>,
        mingo <mingo@...hat.com>, will <will@...nel.org>,
        "boqun.feng" <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [BUG]locking/rwsem: only clean RWSEM_FLAG_HANDOFF when already
 set

On Thu, Nov 11, 2021 at 04:25:56PM -0500, Waiman Long wrote:
> 
> On 11/11/21 16:01, Waiman Long wrote:
> > 
> > On 11/11/21 15:26, Peter Zijlstra wrote:
> > > On Thu, Nov 11, 2021 at 02:36:52PM -0500, Waiman Long wrote:
> > > 
> > > > @@ -434,6 +430,7 @@ static void rwsem_mark_wake(struct
> > > > rw_semaphore *sem,
> > > >               if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
> > > >                   time_after(jiffies, waiter->timeout)) {
> > > >                   adjustment -= RWSEM_FLAG_HANDOFF;
> > > > +                waiter->handoff_set = true;
> > > >                   lockevent_inc(rwsem_rlock_handoff);
> > > >               }
> > > Do we really need this flag? Wouldn't it be the same as waiter-is-first
> > > AND sem-has-handoff ?
> > That is true. The only downside is that we have to read the count first
> > in rwsem_out_nolock_clear_flags(). Since this is not a fast path, it
> > should be OK to do that.
> 
> I just realize that I may still need this flag for writer to determine if it
> should spin after failing to acquire the lock. Or I will have to do extra
> read of count value in the loop. I don't need to use it for writer now.

Maybe it's too late here, but afaict this is right after failing
try_write_lock(), which will have done at least that load you're
interested in, no?

Simply have try_write_lock() update &count or something.