Re: [PATCH] rwlock: remove unneeded subtraction

[Julien Grall <julien@xxxxxxx>]

Hi,

On 15/02/2022 09:39, Roger Pau Monne wrote:
> There's no need to subtract _QR_BIAS from the lock value for storing
> in the local cnts variable in the read lock slow path: the users of
> the value in cnts only care about the writer-related bits and use a
> mask to get the value.
>
> Note that further setting of cnts in rspin_until_writer_unlock already
> do not subtract _QR_BIAS.
The rwlock is a copy of the Linux implementation. So I looked at the 
history to find out why _QR_BIAS was substracted.
It looks like this was done to get better assembly on x86:

commit f9852b74bec0117b888da39d070c323ea1cb7f4c
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date:   Mon Apr 18 01:27:03 2016 +0200

    locking/atomic, arch/qrwlock: Employ atomic_fetch_add_acquire()

    The only reason for the current code is to make GCC emit only the
    "LOCK XADD" instruction on x86 (and not do a pointless extra ADD on
    the result), do so nicer.

    Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
    Acked-by: Waiman Long <waiman.long@xxxxxxx>
    Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
    Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
    Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
    Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
    Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
    Cc: linux-arch@xxxxxxxxxxxxxxx
    Cc: linux-kernel@xxxxxxxxxxxxxxx
    Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>

diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index fec082338668..19248ddf37ce 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -93,7 +93,7 @@ void queued_read_lock_slowpath(struct qrwlock *lock, 
u32 cnts)
         * that accesses can't leak upwards out of our subsequent critical
         * section in the case that the lock is currently held for write.
         */
-       cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts) - _QR_BIAS;
+       cnts = atomic_fetch_add_acquire(_QR_BIAS, &lock->cnts);
        rspin_until_writer_unlock(lock, cnts);

        /*

This is a slowpath, so probably not a concern. But I thought I would 
double check whether the x86 folks are still happy to proceed with that 
in mind.
Cheers,

--
Julien Grall