linux-kernel - Re: [PATCH -next] ipc: make refcounter atomic (was Re: linux-next: Tree for Apr 23 [ Call-Traces: lib/debugobjects.c | kernel/rcupdate.c

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Date:	Wed, 24 Apr 2013 16:38:39 -0700
From:	Davidlohr Bueso <davidlohr.bueso@...com>
To:	sedat.dilek@...il.com
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...riel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Paul McKenney <paul.mckenney@...aro.org>,
	Emmanuel Benisty <benisty.e@...il.com>,
	linux-next <linux-next@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] ipc: make refcounter atomic (was Re: linux-next:
 Tree for Apr 23 [ Call-Traces: lib/debugobjects.c | kernel/rcupdate.c |
 kernel/rcutree.c ])

On Thu, 2013-04-25 at 01:05 +0200, Sedat Dilek wrote:
> On Thu, Apr 25, 2013 at 12:16 AM, Davidlohr Bueso
> <davidlohr.bueso@...com> wrote:
> > From: Davidlohr Bueso <davidlohr.bueso@...com>
> >
> > Sedat reported an issue leading to a NULL dereference in update_queue():
> >
> > [  178.490583] BUG: spinlock bad magic on CPU#1, sh/8066
> > [  178.490595]  lock: 0xffff88008b53ea18, .magic: 6b6b6b6b, .owner: make/8068, .owner_cpu: 3
> > [  178.490599] BUG: unable to handle kernel NULL pointer dereference at           (null)
> > [  178.490608] IP: [<ffffffff812bacd0>] update_queue+0x70/0x210
> > [  178.490610] PGD 0
> > [  178.490612] Oops: 0000 [#1] SMP
> > ...
> > [  178.490704] Call Trace:
> > [  178.490710]  [<ffffffff812baf51>] do_smart_update+0xe1/0x140
> > [  178.490713]  [<ffffffff812bd6e1>] exit_sem+0x2b1/0x350
> > [  178.490718]  [<ffffffff8105de80>] do_exit+0x290/0xa70
> > [  178.490721]  [<ffffffff8105e6f4>] do_group_exit+0x44/0xa0
> > [  178.490724]  [<ffffffff8105e767>] SyS_exit_group+0x17/0x20
> > [  178.490728]  [<ffffffff816ce15d>] system_call_fastpath+0x1a/0x1f
> >
> > Linus pin-pointed the problem to a race in the reference counter. To quote:
> >
> > "That dmesg spew very much implies that the same RCU head got added twice to the RCU
> > freeing list, and the only way that happens is if the refcount goes to
> > zero twice. Which implies that either we increment a zero, or we lack
> > locking and the coherency of the non-atomic access goes away."
> >
> > This patch converts the IPC RCU header's reference counter to atomic_t. The return of
> > ipc_rcu_getref() is modified to inform the callers if it actually succeeded.
> >
> > Now all callers return -EIDRM upon failure and abort the current operation. Two exceptions are
> > in semaphore code where sem_getref_and_unlock() and sem_getref() trigger a warning but proceed
> > to freeing up any held locks.
> >
> > Signed-off-by: Davidlohr Bueso <davidlohr.bueso@...com>
> > Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> > CC: Rik van Riel <riel@...riel.com>
> > CC: Paul McKenney <paul.mckenney@...aro.org>
> > CC: Sedat Dilek <sedat.dilek@...il.com>
> > CC: Emmanuel Benisty <benisty.e@...il.com>
> > CC: Andrew Morton <akpm@...ux-foundation.org>
> 
> Missing my Reported-by ...!

Not trying to take away credit or efforts from you, just wanted you to
reconfirm that *this* actual patch fixes things for you :)

Thanks,
Davidlohr

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