Message-ID: <1423169986.6835.24.camel@stgolabs.net>
Subject: Re: sched: memory corruption on completing completions
From: Davidlohr Bueso <dave@stgolabs.net>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Sasha Levin <sasha.levin@oracle.com>, Waiman Long <Waiman.Long@hp.com>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Ryabinin <a.ryabinin@samsung.com>,
        Dave Jones <davej@codemonkey.org.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Date: Thu, 05 Feb 2015 12:59:46 -0800
In-Reply-To: <CA+55aFwJcG0eAzFJEKD0KVKF2vug-pQpZcoboue9D9SdarsjoQ@mail.gmail.com>
References: <54D2AA16.6030706@oracle.com>
	 <CA+55aFwJcG0eAzFJEKD0KVKF2vug-pQpZcoboue9D9SdarsjoQ@mail.gmail.com>
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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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