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SubjectBKL in do_exit
I was asked by a coworker why the BKL was held in do_exit().  I didn't 
have a good answer for him, so I went digging and found a couple of
Linus quotations on the subject (thank you Google).

> And the do_exit() case should be _trivial_ to fix: almost none of the
> code protected by the kernel lock in the exit path actually needs the
> lock. I suspect you could cut down the kernel lock there to much
> smaller.

> Andrew Morton <akpm@zip.com.au> wrote:
>>That'll be where exit() takes down the tasks's address spaces.
>>zap_page_range(). That's a nasty one.
> No, lock_kernel happens after exit_mm, and in fact I suspect it's not
> really needed at all any more except for the current "sem_exit()". I
> think most everything else is threaded already.
>
> (Hmm.. Maybe disassociate_ctty() too).
>
> So minimizing the BLK footprint in do_exit() should be pretty much
> trivial: all the really interesting stuff should be ok already.

Because of Linus's optimism, I went looking at do_exit(). Here is my
take on it:

lock_kernel();
sem_exit();
// This is questionable, but appears safe to me. Several
// sem_(un)lock()s are done the semid. What else needs to be protected
// here? What was Linus talking about that looks unsafe? Is there a
// chance for another process to be mucking with the current process's
// semaphore lists?

// these hold the task lock and look safe
__exit_files(tsk);
__exit_fs(tsk);

exit_sighand(tsk); // does spin_lock_irq(&tsk->sigmask_lock);
// looks safe
exit_thread(); // nop on most architectures
// FPU cleanup on most others
// looks safe
// there is no locking of the tty. As Linus said, this appears to
// be the bad one now
if (current->leader)
disassociate_ctty(1);
// I can't see why these would need BKL. Looks safe
put_exec_domain(tsk->exec_domain);
if (tsk->binfmt && tsk->binfmt->module)
__MOD_DEC_USE_COUNT(tsk->binfmt->module);
// Does the task lock need to be taken for this?
tsk->exit_code = code;

// Does this need to make sure that none of the relatives exit?
exit_notify();

// BKL implicitly released by calling this, if it is held
schedule();

Can we just hold the BKL around the operations that actually need it?
Is there any other reason to hold it the whole time?

P.S. There are some really fork-happy benchmarks which could see an
improvement from a reduction of lock contention here. It isn't just
theoretical.
--
Dave Hansen
haveblue@us.ibm.com

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