linux-kernel - Re: [PATCH 1/2] btrfs: Implement DRW lock

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Date:   Sat, 8 Jun 2019 18:33:26 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Nikolay Borisov <nborisov@...e.com>
Cc:     linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterz@...radead.org, paulmck@...ux.ibm.com
Subject: Re: [PATCH 1/2] btrfs: Implement DRW lock

On Thu, Jun 06, 2019 at 04:52:18PM +0300, Nikolay Borisov wrote:
> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
> to have multiple readers or multiple writers but not multiple readers
> and writers holding it concurrently. The code is factored out from
> the existing open-coded locking scheme used to exclude pending
> snapshots from nocow writers and vice-versa. Current implementation
> actually favors Readers (that is snapshot creaters) to writers (nocow
> writers of the filesystem).
> 
> Signed-off-by: Nikolay Borisov <nborisov@...e.com>

Interesting!  Thank you for sending this over, Nikolay.

I only have a couple of nits (below) to add:

[...]

> +
> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
> +{
> +	/*
> +	 * Atomic RMW operations imply full barrier, so woken up writers
> +	 * are guaranteed to see the decrement
> +	 */

Not every atomic RMW operations imply a full barrier (as exemplified,
e.g., by the atomic_inc() in btrfs_drw_read_lock()); maybe simply

  s/Atomic RMW operations imply/atomic_dec_and_test() implies/

FYI, checkpatch.pl issues a few warnings on this patch (you may want
to address some of them in the next version).

Thanks,
  Andrea


> +	if (atomic_dec_and_test(&lock->readers))
> +		wake_up(&lock->pending_writers);
> +}
> +
> +