linux-kernel - Re: [PATCH] eCryptfs: Clean up kthread synchronization

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Message-Id: <20080522124142.a850ab0e.akpm@linux-foundation.org>
Date:	Thu, 22 May 2008 12:41:42 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michael Halcrow <mhalcrow@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, prussell@....ibm.com,
	shaggy@...ibm.com, sergeh@...ibm.com
Subject: Re: [PATCH] eCryptfs: Clean up kthread synchronization

On Thu, 22 May 2008 14:31:55 -0500
Michael Halcrow <mhalcrow@...ibm.com> wrote:

> > > +void ecryptfs_destroy_kthread(void)
> > > +{
> > > +	struct ecryptfs_open_req tmp_req;
> > > +	struct ecryptfs_open_req *req;
> > > +
> > > +	mutex_lock(&ecryptfs_kthread_ctl.mux);
> > > +	ecryptfs_kthread_ctl.flags |= ECRYPTFS_KTHREAD_ZOMBIE;
> > > +	list_for_each_entry(req, &ecryptfs_kthread_ctl.req_list,
> > > +			    kthread_ctl_list) {
> > > +		mutex_lock(&req->mux);
> > > +		req->flags |= ECRYPTFS_REQ_ZOMBIE;
> > > +		wake_up_process(req->requesting_task);
> > > +		mutex_unlock(&req->mux);
> > > +	}
> > > +	memset(&tmp_req, 0, sizeof(tmp_req));
> > > +	tmp_req.flags = ECRYPTFS_REQ_ZOMBIE;
> > > +	list_add_tail(&tmp_req.kthread_ctl_list,
> > > +		      &ecryptfs_kthread_ctl.req_list);
> > > +	mutex_unlock(&ecryptfs_kthread_ctl.mux);
> > > +	wake_up(&ecryptfs_kthread_ctl.wait);
> > > +}
> > 
> > eh?  We attach a local variable to a global list and then return?
> > That won't last very long.
> 
> Adding this dummy entry to the list is just my own way of getting the
> kthread to wake up and shut down. This actually works, albeit a little
> ugly. The list and its contents get dropped on the floor at this point
> because (ecryptfs_kthread_ctl.flags & ECRYPTFS_KTHREAD_ZOMBIE). The
> only consumer of this list (the kthread) checks for this flag
> immediately after getting the mux, and if it is there, it just
> exits. The only producer on this list (ecryptfs_privileged_open())
> checks for this flag immediately after getting the mux and bows out if
> it is set. In other words, once this flag is set, the list and its
> contents become untouchable by anything other than
> ecryptfs_destroy_kthread().

Unconvinced.

As soon as ecryptfs_destroy_kthread() returns, tmp_req is destroyed. 
But it remains on ecryptfs_kthread_ctl.req_list.

> 	memset(&tmp_req, 0, sizeof(tmp_req));
> 	tmp_req.flags = ECRYPTFS_REQ_ZOMBIE;
> 	list_add_tail(&tmp_req.kthread_ctl_list,
> 		      &ecryptfs_kthread_ctl.req_list);
> 	mutex_unlock(&ecryptfs_kthread_ctl.mux);
> 	wake_up(&ecryptfs_kthread_ctl.wait);

-> it's dead.

> }

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