linux-kernel - Re: [PATCH] powerpc/qspinlock: Fix deadlock in MCS queue

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Message-ID: <871q29fgn2.fsf@mail.lhotse>
Date: Wed, 28 Aug 2024 13:52:33 +1000
From: Michael Ellerman <mpe@...erman.id.au>
To: "Nysal Jan K.A." <nysal@...ux.ibm.com>, Nicholas Piggin <npiggin@...il.com>
Cc: "Nysal Jan K.A." <nysal@...ux.ibm.com>, Geetika Moolchandani
 <geetika@...ux.ibm.com>, Vaishnavi Bhat <vaish123@...ibm.com>, Jijo
 Varghese <vargjijo@...ibm.com>, Christophe Leroy
 <christophe.leroy@...roup.eu>, Naveen N Rao <naveen@...nel.org>,
 linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/qspinlock: Fix deadlock in MCS queue

"Nysal Jan K.A." <nysal@...ux.ibm.com> writes:
> If an interrupt occurs in queued_spin_lock_slowpath() after we increment
> qnodesp->count and before node->lock is initialized, another CPU might
> see stale lock values in get_tail_qnode(). If the stale lock value happens
> to match the lock on that CPU, then we write to the "next" pointer of
> the wrong qnode. This causes a deadlock as the former CPU, once it becomes
> the head of the MCS queue, will spin indefinitely until it's "next" pointer
> is set by its successor in the queue. This results in lockups similar to
> the following.
...
>
> Thanks to Saket Kumar Bhaskar for help with recreating the issue
>
> Fixes: 84990b169557 ("powerpc/qspinlock: add mcs queueing for contended waiters")
> Cc: stable@...r.kernel.org # v6.2+
> Reported-by: Geetika Moolchandani <geetika@...ux.ibm.com>
> Reported-by: Vaishnavi Bhat <vaish123@...ibm.com>
> Reported-by: Jijo Varghese <vargjijo@...ibm.com>
 
Do we have links for any of these reports?

cheers