Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH] signal: Allow RT tasks to cache one sigqueue struct | Date | Thu, 04 Mar 2021 22:10:03 +0100 |
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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()
and on the free side we have a bunch of callers which do not hold sighand::siglock either. So the cmpxchg() is required.
Thanks,
tglx
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