Re: [PATCH] Avoid that ATA error handling hangs

[Bart Van Assche <Bart.VanAssche@wdc.com>]

On Thu, 2018-02-22 at 04:39 +0000, Bart Van Assche wrote:
> On Thu, 2018-02-22 at 04:19 +0000, Damien Le Moal wrote:
> > It looks OK to me, at least if CONFIG_DEBUG_OBJECTS_RCU_HEAD is turned
> > off since the init_rcu_head() and destroy_rcu_head() functions only care
> > about that.
> > 
> > With rcu head debug turned on, I am not so sure. The object debug code
> > will have references to unused rcu heads left behind for unused scsi
> > cmds, which are indeed dynamically allocated for a device together with
> > requests when the device is initialized, but they are never freed until
> > the device is removed. So "dynamically allocated object", yes, but that
> > does not match the use of the object done in scsi (i.e. alloc before use
> > + free after use).
> 
> Hello Damien,
> 
> Please have a look at the following part of
> Documentation/RCU/Design/Requirements/Requirements.html:
> 
> 	Similarly, statically allocated non-stack <tt>rcu_head</tt>
> 	structures must be initialized with <tt>init_rcu_head()</tt>
> 	and cleaned up with <tt>destroy_rcu_head()</tt>.

And from <linux/rcupdate.h>:

 * rcu_head structures
 * allocated dynamically in the heap or defined statically don't need any
 * initialization.

Bart.