linux-kernel - Re: [PATCH] vfs: make evict() use smp_mb__after_spinlock instead of smp

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Message-ID: <20241113161753.2rtsxuwzgvenwvu4@quack3>
Date: Wed, 13 Nov 2024 17:17:53 +0100
From: Jan Kara <jack@...e.cz>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz,
	jlayton@...nel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs: make evict() use smp_mb__after_spinlock instead of
 smp_mb

On Wed 13-11-24 16:51:03, Mateusz Guzik wrote:
> It literally directly follows a spin_lock() call.
> 
> This whacks an explicit barrier on x86-64.
> 
> Signed-off-by: Mateusz Guzik <mjguzik@...il.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

> This plausibly can go away altogether, but I could not be arsed to
> convince myself that's correct. Individuals willing to put in time are
> welcome :)

AFAICS there's nothing else really guaranteeing the last store to
inode->i_state cannot be reordered up to after the wake up so I think the
barrier should be there.

								Honza
> 
>  fs/inode.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index e5a60084a7a9..b3db1234737f 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -817,7 +817,7 @@ static void evict(struct inode *inode)
>  	 * ___wait_var_event() either sees the bit cleared or
>  	 * waitqueue_active() check in wake_up_var() sees the waiter.
>  	 */
> -	smp_mb();
> +	smp_mb__after_spinlock();
>  	inode_wake_up_bit(inode, __I_NEW);
>  	BUG_ON(inode->i_state != (I_FREEING | I_CLEAR));
>  	spin_unlock(&inode->i_lock);
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR