linux-kernel - Re: sched: memory corruption on completing completions

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Message-ID: <1423169986.6835.24.camel@stgolabs.net>
Date:	Thu, 05 Feb 2015 12:59:46 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Waiman Long <Waiman.Long@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Dave Jones <davej@...emonkey.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Subject: Re: sched: memory corruption on completing completions

On Wed, 2015-02-04 at 16:16 -0800, Linus Torvalds wrote:
> And looking at the arch version, I think the paravirtualized code is crap.
> 
> It does:
> 
>                 prev = *lock;
>                 add_smp(&lock->tickets.head, TICKET_LOCK_INC);
> 
>                 /* add_smp() is a full mb() */
> 
>                 if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG))
>                         __ticket_unlock_slowpath(lock, prev);
> 
> 
> which is *exactly* the kind of things you cannot do with spinlocks,
> because after you've done the "add_smp()" and released the spinlock
> for the fast-path, you can't access the spinlock any more.  Exactly
> because a fast-path lock migth come in, and release the whole data
> structure.
> 
> As usual, the paravirt code is a horribly buggy heap of crud. Film at 11.

Per http://lwn.net/Articles/495597/ which clearly describes the intent
of the slowpath unlocking. Cc'ing Raghavendra.

Thanks,
Davidlohr

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