Re: [PATCH] libfc: replace 'rp_mutex' with 'rp_lock'

[Hannes Reinecke <hare@suse.de>]

On 05/11/2016 04:44 PM, Bart Van Assche wrote:
> On 05/10/16 23:07, Hannes Reinecke wrote:
>> On 05/11/2016 07:49 AM, Hannes Reinecke wrote:
>> RIP: 0010:[<ffffffffa00b2adb>]  [<ffffffffa00b2adb>]
>> fc_rport_lookup+0x4b/0x70 [libfc]
>> Call Trace:
>>    [<ffffffffa00b2e17>] fc_rport_create+0x17/0x1b0 [libfc]
>>    [<ffffffffa00a9f81>] fc_disc_recv_req+0x261/0x480 [libfc]
>>    [<ffffffffa00b1008>] fc_lport_recv_els_req+0x68/0x130 [libfc]
>>    [<ffffffffa00afd5a>] fc_lport_recv_req+0x9a/0xf0 [libfc]
>>    [<ffffffffa00e8333>] fnic_handle_frame+0x63/0xd0 [fnic]
>>    [<ffffffff8106fd52>] process_one_work+0x172/0x420
>>    [<ffffffff810709ca>] worker_thread+0x11a/0x3c0
>>    [<ffffffff81077344>] kthread+0xb4/0xc0
>>    [<ffffffff81521318>] ret_from_fork+0x58/0x90
>
> Hello Hannes,
>
> Thanks for sharing this information. fc_disc_recv_req() protects the
> fc_rport_create() call via a mutex (disc_mutex). Since a mutex_lock()
> call may sleep it can trigger the start of an RCU grace period. I think
> this may result in freeing of an rport while fc_rport_lookup() is
> examining it. Have you already considered to add a
> rcu_read_lock()/rcu_read_unlock() pair in fc_rport_lookup()?
>
No, I haven't so far.
This issue is hard to trigger, and I've only got the customer report to 
go by.

Also, when using an rcu_read_lock() here one probably needs to revisit 
the entire locking structure in libfc:
rport list is an RCU-proctected list, so in principle one only needs to 
hold the rport mutex when _assigning_ new rports, and _could_ drop the 
mutex usage for a simple lookup.
But this really needs some more thought and testing, so I haven't 
attempted it.

Also, with your suggestion I would need to take the mutex_lock _and_ 
call rcu_read_lock(), which just looks wrong.
Hence I prefer to use the spin_lock method (which, incidentally, is also 
suggested in the RCU documentation).

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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