linux-kernel - Re: [PATCH v2] sg: O_EXCL and other lock handling

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Date:	Thu, 31 Oct 2013 08:56:53 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Douglas Gilbert <dgilbert@...erlog.com>
Cc:	SCSI development list <linux-scsi@...r.kernel.org>,
	James Bottomley <james.bottomley@...senpartnership.com>,
	vaughan <vaughan.cao@...cle.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sg: O_EXCL and other lock handling

> +	struct semaphore or_sem;  /* protect co-incident opens and releases */

Seems like this should be a mutex.

> sfds_list_empty(Sg_device *sdp)
>  {
>  	unsigned long flags;
>  	int ret;
>  
> +	spin_lock_irqsave(&sdp->sfd_lock, flags);
> +	ret = list_empty(&sdp->sfds);
> +	spin_unlock_irqrestore(&sdp->sfd_lock, flags);
>  	return ret;

Protecting just a list_empty check with a local will give you racy
results.  Seems like you should take the look over the check and the
resulting action that modifies the list.  That'd also mean replacing the
wait_event* calls with open coded prepare_wait / finish_wait loops.

> +	down(&sdp->or_sem);
> +	alone = sfds_list_empty(sdp);
> +	if ((flags & O_EXCL) && (O_RDONLY == (flags & O_ACCMODE))) {
> +		retval = -EPERM; /* Don't allow O_EXCL with read only access */
> +		goto error_out;
> +	}

Seems like the pure flags check should move to the beginning of the
function before taking any locks.

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