linux-kernel - Re: [PATCH] x86 spinlock: Fix memory corruption on completing completions

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Date:	Tue, 10 Feb 2015 15:24:17 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Peter Anvin <hpa@...or.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <waiman.long@...com>,
	Dave Jones <davej@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Jason Wang <jasowang@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	xen-devel@...ts.xenproject.org, Rik van Riel <riel@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Ryabinin <a.ryabinin@...sung.com>
Subject: Re: [PATCH] x86 spinlock: Fix memory corruption on completing
	completions

On 02/10, Denys Vlasenko wrote:
>
> # define HEAD_MASK (TICKET_SLOWPATH_FLAG-1)
>
> ...
> unlock_again:
>
> val = xadd((&lock->ticket.head_tail, TICKET_LOCK_INC);
> if (unlikely(!(val & HEAD_MASK))) {
>     /* overflow. we inadvertently incremented the tail word.
>      * tail's lsb is TICKET_SLOWPATH_FLAG.
>      * Increment inverted this bit, fix it up.
>      * (inc _may_ have messed up tail counter too,
>      * will deal with it after kick.)
>     */
>     val ^= TICKET_SLOWPATH_FLAG;
> }
>
> if (unlikely(val & TICKET_SLOWPATH_FLAG)) {
>     ...kick the waiting task...
>
>    val -= TICKET_SLOWPATH_FLAG;
>    if (unlikely(!(val & HEAD_MASK))) {
>         /* overflow. we inadvertently incremented tail word, *and*
>          * TICKET_SLOWPATH_FLAG was set, increment overflowed
>          * that bit too and incremented tail counter.
>          * This means we (inadvertently) taking the lock again!
>          * Oh well. Take it, and unlock it again...
>          */
>         while (1) {
>             if (READ_ONCE(lock->tickets.head) != TICKET_TAIL(val))
>                 cpu_relax();
>         }
>         goto unlock_again;
> }
>
>
> Granted, this looks ugly.

complicated ;)

But "Take it, and unlock it again" simply can't work, this can deadlock.
Note that unlock() can be called after successful try_lock(). And other
problems with lock-ordering, like

	lock(X);
	lock(Y);
	
	unlock(X);

Oleg.

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