linux-kernel - Re: [PATCH] net/p9/trans_fd.c: fix double list

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Message-ID: <20180724101932.GA17454@nautica>
Date:   Tue, 24 Jul 2018 12:19:32 +0200
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Tomas Bortoli <tomasbortoli@...il.com>
Cc:     jiangyiwen <jiangyiwen@...wei.com>, davem@...emloft.net,
        v9fs-developer@...ts.sourceforge.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller@...glegroups.com
Subject: Re: [PATCH] net/p9/trans_fd.c: fix double list_del()

Tomas Bortoli wrote on Tue, Jul 24, 2018:
> >> @@ -228,6 +226,7 @@ static void p9_conn_cancel(struct p9_conn *m, int err)
> >>  			req->t_err = err;
> >>  		p9_client_cb(m->client, req, REQ_STATUS_ERROR);
> >>  	}
> >> +	spin_unlock(&m->client->lock);
> > 
> > If you want to expand the ranges of client->lock, the cancel_list will not
> > be necessary, you can optimize this code.
> > 
> 
> Unfortunately, not. Moving the spin_lock() before the for makes the
> crash appear again. This because the calls to list_move() in the for
> before delete all the elements from req->req_list, so the list is empty,
> another call to list_del() would trigger a double del.
> That's why we hold the lock to update the status of all those requests..
> otherwise we have again the race with p9_fd_cancel().

What (I think) he meant is that since you're holding the lock all the
way, you don't need to transfer all the items to a temporary list to
loop on it immediately afterwards, but you could call the client cb
directly.

I'm personally not a fan of this approach as that would duplicate the
code, even if the loop isn't big...

This code is only called at disconnect time so I think using the extra
list doesn't hurt anyone; but as usual do what you feel is better; I
don't mind much either way.

-- 
Dominique Martinet