linux-kernel - Re: [PATCH 2/2] [PATCH] sched: Add smp

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Date:	Fri, 20 Feb 2015 19:28:16 +0100
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Kirill Tkhai <ktkhai@...allels.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles

Hi Oleg,

my example was bad, let's continue with your example.

And: If sem_lock() needs another smp_xmb(), then we must add it:
Some apps do not have a user space hot path, i.e. it seems that on some 
setups, we have millions of calls per second.
If there is a race, then it will happen.

I've tried to merge your example:
 >
 > int X = 0, Y = 0;
 >
 > void func(void)
 > {
 >     bool ll = rand();
 >
 >     if (ll) {
 >         spin_lock(&local);
 >         if (!spin_is_locked(&global))
 >             goto done;
 >         spin_unlock(&local);
 >     }
 >     ll = false;
 >     spin_lock(&global);
 >     spin_unlock_wait(&local);
 > done:
 >     smp_rmb(); <<<<<<<<<<<<<<<
 >     BUG_ON(X != Y);
 >
 >     ++X; ++Y;
 >
 >     if (ll)
 >         spin_unlock(&local);
 >     else
 >         spin_unlock(&global);
 > }
I agree, we need the smp_rmb().
I'll write a patch.

> We need the full barrier to serialize STORE's as well, but probably we can
> rely on control dependancy and thus we only need rmb().
Do we need a full barrier or not?

I don't manage to create a proper line of reasoning.
--
     Manfred
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