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SubjectRe: [PATCH] jump label: close race in jump_label_inc() vs. jump_label_dec()
On Wed, Jan 04, 2012 at 10:32:37AM -0500, Jason Baron wrote:
> The previous fix to ensure that jump_label_inc() does not return until the jump
> is completely patched, opened up a race in the inc/dec path. The scenario is:
>
> key->enabled = 0;
>
> CPU 0 CPU 1
> ----- -----
>
> jump_label_inc(): jump_label_dec():
>
> 1) if (atomic_read(&key->enabled) == 0)
> jump_label_update(key, JUMP_LABEL_ENABLE);
>
> 2) if (!atomic_dec_and_mutex_lock(&key->enabled, &jump_label_mutex))
> return;
>
> 3) atomic_inc(&key->enabled);
>
> So now, key->enabled = 0, but the jump has been enabled, which is an invalid
> state.
>
Isn't it an indication of a higher level bug if jump_label_dec() is
called on a disabled jump label? In other words isn't key->enabled == -1
invalid sate by itself? I do not see how the call sequence above can
happen with perf events for instance. jump_label_dec() is called on
event destruction and if key->enabled = 0 then there is no events to
destroy.

--
Gleb.


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