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SubjectRe: [PATCH] signal: Allow RT tasks to cache one sigqueue struct
Date
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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