On 03/04, Thomas Gleixner wrote: > > On Wed, Mar 03 2021 at 16:37, Oleg Nesterov wrote: > > On 03/03, Sebastian Andrzej Siewior wrote: > >> > >> +static struct sigqueue *sigqueue_from_cache(struct task_struct *t) > >> +{ > >> + struct sigqueue *q = t->sigqueue_cache; > >> + > >> + if (q && cmpxchg(&t->sigqueue_cache, q, NULL) == q) > >> + return q; > >> + return NULL; > >> +} > >> + > >> +static bool sigqueue_add_cache(struct task_struct *t, struct sigqueue *q) > >> +{ > >> + if (!t->sigqueue_cache && cmpxchg(&t->sigqueue_cache, NULL, q) == NULL) > >> + return true; > >> + return false; > >> +} > > > > Do we really need cmpxchg? It seems they are always called with > > spinlock held. > > With which spinlock held? > > __send_signal() <- sighand::siglock held > __sigqueue_alloc() > > alloc_posix_timer() > sigqueue_alloc() <- No lock held > __sigqueue_alloc()
In the last case "fromslab" is true, sigqueue_from_cache() won't be called. > and on the free side we have a bunch of callers which do not hold > sighand::siglock either. Where? Oleg.