linux-kernel - Re: [PATCH -v6 2/3] lib, Make gen

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Date:	Mon, 29 Nov 2010 13:11:37 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Huang Ying <ying.huang@...el.com>
Cc:	Len Brown <lenb@...nel.org>, linux-kernel@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>, linux-acpi@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH -v6 2/3] lib, Make gen_pool memory allocator lockless

On Mon, 2010-11-29 at 15:03 +0800, Huang Ying wrote:
> This version of the gen_pool memory allocator supports lockless
> operation.
> 
> This makes it safe to use in NMI handlers and other special
> unblockable contexts that could otherwise deadlock on locks.  This is
> implemented by using atomic operations and retries on any conflicts.
> The disadvantage is that there may be livelocks in extreme cases.  For
> better scalability, one gen_pool allocator can be used for each CPU.
> 
> The lockless operation only works if there is enough memory available.
> If new memory is added to the pool a lock has to be still taken.  So
> any user relying on locklessness has to ensure that sufficient memory
> is preallocated.
> 
> The basic atomic operation of this allocator is cmpxchg on long.  On
> architectures that don't have NMI-safe cmpxchg implementation, a
> spin_trylock_irqsave based fallback is used for gen_pool_alloc, so it
> can be used in NMI handler safely.  But gen_pool_free can not be used
> in NMI handler in these architectures, because memory free can not
> fail. 

I still don't see a reason to merge this.

It makes the genalloc thing slower for every other user (more LOCK'ed
ops) and there is no new user presented in this series.


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