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SubjectRe: [PATCH] Fast path context switch - microoptimize FPU reload
Andi wrote:

>We don't need the lock prefix to test our local thread flags state.
>Unfortunately test_thread_flag currently always uses test_bit which
>has a LOCK on SMP, but that's unnecessary. LOCK is costly on P4,
>so it's a good idea to avoid it.

No, LOCK is required: the TIF_ flags word is also used for signal
delivery - writing without lock could corrupt state.

What about moving TIF_USEDFPU from the thread_info into
task_struct->flags? This flag word is only accessed by "current", no
special atomicity requirements.


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