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

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Date:	Tue, 10 Feb 2015 14:18:05 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	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>, Oleg Nesterov <oleg@...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 Tue, Feb 10, 2015 at 10:30 AM, Raghavendra K T
<raghavendra.kt@...ux.vnet.ibm.com> wrote:
> On 02/10/2015 06:23 AM, Linus Torvalds wrote:
>>          add_smp(&lock->tickets.head, TICKET_LOCK_INC);
>>          if (READ_ONCE(lock->tickets.tail) & TICKET_SLOWPATH_FLAG) ..
>>
>> into something like
>>
>>          val = xadd((&lock->ticket.head_tail, TICKET_LOCK_INC <<
>> TICKET_SHIFT);
>>          if (unlikely(val & TICKET_SLOWPATH_FLAG)) ...
>>
>> would be the right thing to do. Somebody should just check that I got
>> that shift right, and that the tail is in the high bytes (head really
>> needs to be high to work, if it's in the low byte(s) the xadd would
>> overflow from head into tail which would be wrong).
>
> Unfortunately xadd could result in head overflow as tail is high.

xadd can overflow, but is this really a problem?

# 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.
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