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SubjectRe: [PATCH 3/3] posix timers: use SIGQUEUE_CANCELLED when the timer is destroyed


On Sun, 18 May 2008, Oleg Nesterov wrote:
>
> Initially, I did
>
> q->flags |= SIGQUEUE_CANCELLED;
> spin_lock_irqsave(lock, flags);
> q->flags &= ~SIGQUEUE_PREALLOC;
>
> to document the fact that SIGQUEUE_CANCELLED can be set lockless, but
> then "optimized" the code, couldn't help myself... Besides, the code
> above looks really confusing without the fat comment.

Oh, and the above is *wrong*.

Why?

Becayse if SIGQUEUE_PREALLOC setting needs the lock, then setting any
*other* bit in that word will also need the lock!

That's because

q->flags |= SIGQUEUE_CANCELLED;

writes those other bits too - admittedly with the value they were read
just before, but if it races with something setting SIGQUEUE_PREALLOC that
doesn't matter - the newly written version will simply be wrong.

So the rule is that if one bit of a word needs locking, then they *all*
do.

(On alpha, this is true even for whole bytes or shortwords - because a
byte/shortword write is actually "read word, update byte/short, write
word" sequence on older CPU's. So you cannot do atomic byte updates, and
need to use locks).

Linus


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