linux-kernel - Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

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Date:	Tue, 22 Apr 2008 08:01:22 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	npiggin@...e.de
Subject: Re: [PATCH 1/11] Add generic helpers for arch IPI function calls



On Tue, 22 Apr 2008, Jens Axboe wrote:
> > 
> > You forgot to free the "data" here? The waiter must also free the object, 
> > since now the callee does not.
> 
> The ipi interrupt handler does that, see kfree() in
> generic_smp_call_function_single_interrupt() or call_func_data_free() in
> generic_smp_call_function_interrupt().

Hell no, it does *not*.

Doing that for the waiting case would be a *huge* bug, since the waiter 
needs to wait until the flag is clear - and if the waitee free's the 
allocation, that will never happen.

So the rule *must* be:
 - waiter frees
 - ipi interrupt frees non-waiting ones.
because anything else cannot work.

And you must have known that, because the code you pointed me to does 
*not* free the data at all. It just clears the FLAG_WAIT flag:

	+               if (data->csd.flags & CSD_FLAG_WAIT) {
	+                       smp_wmb();
	+                       data->csd.flags &= ~CSD_FLAG_WAIT;
	+               } else
	+                       call_func_data_free(data);

So please think about this some more. 

		Linus
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