netdev - Re: [PATCH] net/core/sock.c remove extra wakeup

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Date:	Sun, 01 Jun 2008 23:03:41 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"Patrick Mullaney" <pmullaney@...ell.com>
Cc:	"Gregory Haskins" <GHaskins@...ell.com>, <davem@...emloft.net>,
	<netdev@...r.kernel.org>
Subject: Re: [PATCH] net/core/sock.c remove extra wakeup

"Patrick Mullaney" <pmullaney@...ell.com> writes:
>
> Would a better approach be?
>
> 1) clear the bit when the waiter returns(bottom of sock_wait_for_wmem)
> 2) add a new wait-queue to the socket to separate the writers
> 3) another idea?

A large reason this is tricky is because it is essentially lockless. So
alternative would be:

4) you add a spinlock which is always taken around the various condition
checks and bit manipulation. Not 100% sure if it would be bad to 
general SMP scalability (you would need to ensure that the bouncing
lock cache line isn't a performance problem), but at least it would 
be much safer and makes it much easier to verify your change is correct.

Actually the wait queue already has such a lock, so it might not be
that expensive if you switch the wait queue to a lockless version.

-Andi

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