Messages in this thread |  | | Subject | Re: [PATCH] Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()" | From | Manfred Spraul <> | Date | Mon, 16 Dec 2019 20:04:53 +0100 |
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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.
> [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> > --- > 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++) {
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