linux-kernel - [PATCH] ipc,sem block sem_lock on sma->lock during sma initialization

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Date:	Fri, 21 Nov 2014 14:52:26 -0500
From:	Rik van Riel <riel@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Manfred Spraul <manfred@...orfullife.com>,
	Davidlohr Bueso <davidlohr@...com>,
	Rafael Aquini <aquini@...hat.com>
Subject: [PATCH] ipc,sem block sem_lock on sma->lock during sma
 initialization

When manipulating just one semaphore with semop, sem_lock only takes that
single semaphore's lock. This creates a problem during initialization of
the semaphore array, when the data structures used by sem_lock have not
been set up yet. The sma->lock is already held by newary, and we just
have to make sure everything else waits on that lock during initialization.

Luckily it is easy to make sem_lock wait on the sma->lock, by pretending
there is a complex operation in progress while the sma is being initialized.

The newary function already zeroes sma->complex_count before unlocking
the sma->lock.

Signed-off-by: Rik van Riel <riel@...hat.com>
---
 ipc/sem.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/ipc/sem.c b/ipc/sem.c
index 454f6c6..1823160 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -507,6 +507,9 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
 		return retval;
 	}

+	/* Ensures sem_lock waits on &sma->lock until sma is ready. */
+	sma->complex_count = 1;
+
 	id = ipc_addid(&sem_ids(ns), &sma->sem_perm, ns->sc_semmni);
 	if (id < 0) {
 		ipc_rcu_putref(sma, sem_rcu_free);
--
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