linux-kernel - Re: [PATCH 4/11] LTTng-core 0.5.108 : core

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Date:	Thu, 14 Sep 2006 11:37:43 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...l.org>, Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tom Zanussi <zanussi@...ibm.com>, ltt-dev@...fik.org,
	Michel Dagenais <michel.dagenais@...ymtl.ca>
Subject: Re: [PATCH 4/11] LTTng-core 0.5.108 : core

* Serge E. Hallyn (serue@...ibm.com) wrote:
> Hi,
> 
> I'm wondering why this is safe:
> 
> you grab references to the object which may be deleted after
> you drop the transport_list_lock at the top of this block.  Since
> a later patch shows the unregister being called right before the
> owning module is unloaded, that seems awefuly dangerous.
> 
> Is there some other magic going on making this safe?
> 

The ltt_traces_sem mutex is intended to make this safe. However, the transport
separation patch, contributed recently, uses its own transport_list_lock, which
seems to be broken.

I will fix it by using ltt_traces_sem around :


        down(&ltt_traces_sem);
        list_for_each_entry(tran, &ltt_transport_list, node) {
                if (!strcmp(tran->name, trace_type)) {
                        transport = tran;
                        break;
                }
        }

        if (!transport) {
                err = EINVAL;
                printk(KERN_ERR "LTT : Transport %s is not present.\n", trace_type);
                goto trace_error;
        }

        if(!try_module_get(transport->owner)) {
                err = ENODEV;
                printk(KERN_ERR "LTT : Can't lock transport module.\n");
                goto trace_error;
        }
        up(&ltt_traces_sem);

And change the transport_list_lock for ltt_traces_sem everywhere else.

Thanks for spotting this bug,

Mathieu


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