Re: [PATCH] Change synchronize_rcu() in scsi_device_quiesce() into synchronize_sched()

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

On Mon, 2018-03-19 at 08:21 -0700, tj@kernel.org wrote:
> On Mon, Mar 19, 2018 at 03:16:07PM +0000, Bart Van Assche wrote:
> > As explained in the comment in scsi_device_quiesce(), the effect of
> > blk_set_preempt_only() must be visible for percpu_ref_tryget() calls that
> > occur after the queue unfreeze triggered by scsi_device_quiesce(). Hence the
> > RCU read lock calls in blk_queue_enter(). Since these RCU read lock calls
> > surround a function call that uses rcu_read_lock_sched(), namely
> > percpu_ref_tryget_live(), we have to use sched-RCU in both blk_queue_enter()
> > and scsi_device_quiesce().
> 
> Let's please not depend on rcu_read_lock_sched() in
> percpu_ref_tryget_live().  That's an implementation detail.  If the
> code needs RCU based synchronization, it should perform them
> explicitly.

Hello Tejun,

The algorithm explained above does not depend on sched-rcu. But because
percpu_ref_tryget_live() uses sched-rcu and because we need to add an RCU lock
around that call we are forced to use sched-rcu. I hope this makes it clear.

Bart.