linux-kernel - Re: [PATCH] Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit

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Date:   Tue, 17 Dec 2019 18:17:45 -0300
From:   "Herton R. Krzesinski" <herton@...hat.com>
To:     Manfred Spraul <manfred@...orfullife.com>
Cc:     Ioanna Alifieraki <ioanna-maria.alifieraki@...onical.com>,
        akpm@...ux-foundation.org, arnd@...db.de, catalin.marinas@....com,
        malat@...ian.org, joel@...lfernandes.org, gustavo@...eddedor.com,
        linux-kernel@...r.kernel.org, jay.vosburgh@...onical.com,
        ioanna.alifieraki@...il.com
Subject: Re: [PATCH] Revert "ipc,sem: remove uneeded sem_undo_list lock usage
 in exit_sem()"

On Mon, Dec 16, 2019 at 08:04:53PM +0100, Manfred Spraul wrote:
> Hi Ioanna,
> 
> On 12/11/19 8:13 PM, Ioanna Alifieraki wrote:
> > This reverts commit a97955844807e327df11aa33869009d14d6b7de0.
> > 
> > Commit a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage
> > in exit_sem()") removes a lock that is needed.
> 
> Yes, you are right, the lock is needed.
> 
> The documentation is already correct:
> 
> sem_undo_list.list_proc: undo_list->lock for write.
> 
> [...]
> > Removing elements from list_id is safe for both exit_sem() and freeary()
> > due to sem_lock().  Removing elements from list_proc is not safe;
> 
> Correct, removing elements is not safe.
> 
> Removing one element would be ok, as we hold sem_lock.
> 
> But if there are two elements, then we don't hold sem_lock for the 2nd
> element, and thus the list is corrupted.

I think that's what I overlooked/missed back then, sorry for the bug.

> 
> > [1] https://bugzilla.redhat.com/show_bug.cgi?id=1694779
> > 
> > Fixes: a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()")
> > Signed-off-by: Ioanna Alifieraki <ioanna-maria.alifieraki@...onical.com>
> Acked-by: Manfred Spraul <manfred@...orfullife.com>

Acked-by: Herton R. Krzesinski <herton@...hat.com>

> > ---
> >   ipc/sem.c | 6 ++----
> >   1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > diff --git a/ipc/sem.c b/ipc/sem.c
> > index ec97a7072413..fe12ea8dd2b3 100644
> > --- a/ipc/sem.c
> > +++ b/ipc/sem.c
> > @@ -2368,11 +2368,9 @@ void exit_sem(struct task_struct *tsk)
> >   		ipc_assert_locked_object(&sma->sem_perm);
> >   		list_del(&un->list_id);
> > -		/* we are the last process using this ulp, acquiring ulp->lock
> > -		 * isn't required. Besides that, we are also protected against
> > -		 * IPC_RMID as we hold sma->sem_perm lock now
> > -		 */
> > +		spin_lock(&ulp->lock);
> >   		list_del_rcu(&un->list_proc);
> > +		spin_unlock(&ulp->lock);
> >   		/* perform adjustments registered in un */
> >   		for (i = 0; i < sma->sem_nsems; i++) {
> 
> 

-- 
[]'s
Herton