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[2003:d9:9706:6200:4ee5:de74:b236:a15c]) by smtp.googlemail.com with ESMTPSA id w20sm308960wmk.34.2019.12.16.11.04.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 16 Dec 2019 11:04:54 -0800 (PST) Subject: Re: [PATCH] Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()" To: Ioanna Alifieraki , akpm@linux-foundation.org, herton@redhat.com, arnd@arndb.de, catalin.marinas@arm.com, malat@debian.org, joel@joelfernandes.org, gustavo@embeddedor.com, linux-kernel@vger.kernel.org, jay.vosburgh@canonical.com, ioanna.alifieraki@gmail.com References: <20191211191318.11860-1-ioanna-maria.alifieraki@canonical.com> From: Manfred Spraul Message-ID: Date: Mon, 16 Dec 2019 20:04:53 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: <20191211191318.11860-1-ioanna-maria.alifieraki@canonical.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Ioanna, On 12/11/19 8:13 PM, Ioanna Alifieraki wrote: > This reverts commit a97955844807e327df11aa33869009d14d6b7de0. > > Commit a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage > in exit_sem()") removes a lock that is needed. Yes, you are right, the lock is needed. The documentation is already correct: sem_undo_list.list_proc: undo_list->lock for write. [...] > Removing elements from list_id is safe for both exit_sem() and freeary() > due to sem_lock(). Removing elements from list_proc is not safe; Correct, removing elements is not safe. Removing one element would be ok, as we hold sem_lock. But if there are two elements, then we don't hold sem_lock for the 2nd element, and thus the list is corrupted. > [1] https://bugzilla.redhat.com/show_bug.cgi?id=1694779 > > Fixes: a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()") > Signed-off-by: Ioanna Alifieraki Acked-by: Manfred Spraul > --- > ipc/sem.c | 6 ++---- > 1 file changed, 2 insertions(+), 4 deletions(-) > > diff --git a/ipc/sem.c b/ipc/sem.c > index ec97a7072413..fe12ea8dd2b3 100644 > --- a/ipc/sem.c > +++ b/ipc/sem.c > @@ -2368,11 +2368,9 @@ void exit_sem(struct task_struct *tsk) > ipc_assert_locked_object(&sma->sem_perm); > list_del(&un->list_id); > > - /* we are the last process using this ulp, acquiring ulp->lock > - * isn't required. Besides that, we are also protected against > - * IPC_RMID as we hold sma->sem_perm lock now > - */ > + spin_lock(&ulp->lock); > list_del_rcu(&un->list_proc); > + spin_unlock(&ulp->lock); > > /* perform adjustments registered in un */ > for (i = 0; i < sma->sem_nsems; i++) {