On Tue, 26 Mar 2013 10:59:27 -0700 Davidlohr Bueso <davidlohr.bu...@hp.com>
wrote:
> On Mon, 2013-03-25 at 20:47 +0700, Emmanuel Benisty wrote:
> > On Mon, Mar 25, 2013 at 12:10 AM, Linus Torvalds
> > <torva...@linux-foundation.org> wrote:
> > > And you never see this problem without Rik's patches?
> >
> > No, never.
> >
> > > Could you bisect
> > > *which* patch it starts with? Are the first four ones ok (the moving
> > > of the locking around, but without the fine-grained ones), for
> > > example?
> >
> > With the first four patches only, I got some X server freeze (just tried
> > once).
>
> Going over the code again, I found a potential recursive spinlock scenario.
> Andrew, if you have no objections, please queue this up.
>
> Thanks.
>
> ---8<---
>
> From: Davidlohr Bueso <davidlohr.bu...@hp.com>
> Subject: [PATCH] ipc, sem: prevent possible deadlock
>
> In semctl_main(), when cmd == GETALL, we're locking
> sma->sem_perm.lock (through sem_lock_and_putref), yet
> after the conditional, we lock it again.
> Unlock sma right after exiting the conditional.
>
> Signed-off-by: Davidlohr Bueso <davidlohr.bu...@hp.com>
> ---
> ipc/sem.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/ipc/sem.c b/ipc/sem.c
> index 1a2913d..f257afe 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -1243,6 +1243,7 @@ static int semctl_main(struct ipc_namespace *ns, int
> semid, int semnum,
> err = -EIDRM;
> goto out_free;
> }
> + sem_unlock(sma, -1);
> }
>
> sem_lock(sma, NULL, -1);
Looks right.
Do we need the locking at all? What does it actually do?
sem_lock_and_putref(sma);
if (sma->sem_perm.deleted) {
sem_unlock(sma, -1);
err = -EIDRM;
goto out_free;
}
sem_unlock(sma, -1);
We're taking the lock, testing an int and then dropping the lock.
What's the point in that?
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