Messages in this thread |  | | Date | Tue, 17 Dec 2019 18:17:45 -0300 | From | "Herton R. Krzesinski" <> | Subject | Re: [PATCH] Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()" |
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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@canonical.com> > Acked-by: Manfred Spraul <manfred@colorfullife.com>
Acked-by: Herton R. Krzesinski <herton@redhat.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
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