linux-kernel - Re: [patch 4/4] ipc: sem optimise simple operations

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Message-ID: <20090812044837.GC5330@wotan.suse.de>
Date:	Wed, 12 Aug 2009 06:48:37 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Manfred Spraul <manfred@...orfullife.com>,
	Nadia Derbey <Nadia.Derbey@...l.net>,
	Pierre Peiffer <peifferp@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 4/4] ipc: sem optimise simple operations

On Wed, Aug 12, 2009 at 12:07:11AM +0400, Cyrill Gorcunov wrote:
> [npiggin@...e.de - Tue, Aug 11, 2009 at 09:09:06PM +1000]
> ...
> | +static void update_queue_simple(struct sem_array *sma, ushort semnum)
> | +{
> | +	if (unlikely(sma->complex_count)) {
> | +		update_queue(sma);
> | +	} else {
> | +		struct sem *sem;
> | +
> | +		sem = &sma->sem_base[semnum];
> | +		if (sem->semval > 0)
> | +			update_negv_queue(sma, sem);
> | +		if (sem->semval == 0)
> | +			update_zero_queue(sma, sem);
> | +	}
> | +}
> | +
> ...
> 
> Hi Nick,
> 
> mostly probably miss something but can't we trgigger BUG_ON at updating
> zero queue if semaphore was created with undo list and via new operation
> reached -ERANGE on undo value?
> 
> Again, I could be missing something or plain wrong. Just a thought.

Hi Cyrill,

Thanks for looking... Hmm, you mean BUG_ON(error) due to try_atomic_semop
returning -ERANGE? I think it should not be possible because it should
prevent any operation from bringing the undo list to -ERANGE so then any
operation which does not modify the sem value should not go out of range
I think.

(I think it would be a bug if we ever return -ERANGE for a wait-for-zero
operation).

Thanks,
Nick
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