Re: [PATCH 6.6] Revert "dm-crypt, dm-verity: disable tasklets"

[Li Lingfeng <lilingfeng3@xxxxxxxxxx>]

Hi

I'm having difficulty understanding "Workqueues and ksoftirqd may be 
scheduled arbitrarily".
This is my understanding:
kcryptd_queue_crypt
 tasklet_schedule
  __tasklet_schedule
   __tasklet_schedule_common
    raise_softirq_irqoff
     wakeup_softirqd
      wake_up_process // ksoftirqd

run_ksoftirqd
 __do_softirq
  softirq_handle_begin
   __local_bh_disable_ip // Turn off preemption
<---------- [1] ---------->
  tasklet_action // h->action
   tasklet_action_common
    tasklet_trylock
     kcryptd_crypt_tasklet // t->func(t->data)
     ...
      queue_work(cc->io_queue, &io->work)
<---------- [2] ---------->
    tasklet_unlock

// workqueue process
kcryptd_io_bio_endio
 ...
 // free tasklet_struct

Since preemption has been turned off at [1], I'm confused about how the 
CPU can be scheduled out to do work first at [2].
Would you mind explaining it to me?

Thanks

在 2024/4/11 20:37, Mikulas Patocka 写道:
> Hi
>
> I NACK this.
>
>
> On Thu, 11 Apr 2024, Li Lingfeng wrote:
>
> > This reverts commit 5735a2671ffb70ea29ca83969fe01316ee2ed6fc which is
> > commit 0a9bab391e336489169b95cb0d4553d921302189 upstream.
> >
> > Tasklet is thought to cause memory corruption [1], so it was disabled in
> > dm-crypt and dm-verity. However, memory corruption may not happen since
> > cc->io_queue is created without WQ_UNBOUND [2].
> > Revert commit 5735a2671ffb ("dm-crypt, dm-verity: disable tasklets") to
> > bring tasklet back.
> >
> > [1] https://lore.kernel.org/all/d390d7ee-f142-44d3-822a-87949e14608b@xxxxxxx/T/
> > [2] https://lore.kernel.org/all/4d331659-badd-749d-fba1-271543631a8a@xxxxxxxxxx/
> Regarding [2] - if you add mdelay, you can make the race condition less
> reproducible, but it is not a proper fix and the race condition stays
> there unfixed.
>
> Workqueues and ksoftirqd may be scheduled arbitrarily, there is no
> guarantee that they will be executed in a particular order, even if they
> are executed on the same CPU.
>
> Mikulas
>
> > Signed-off-by: Li Lingfeng <lilingfeng3@xxxxxxxxxx>
> > ---
> >   drivers/md/dm-crypt.c         | 38 +++++++++++++++++++++++++++++++++--
> >   drivers/md/dm-verity-target.c | 26 ++++++++++++++++++++++--
> >   drivers/md/dm-verity.h        |  1 +
> >   3 files changed, 61 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
> > index aa6bb5b4704b..a60d91d02e28 100644
> > --- a/drivers/md/dm-crypt.c
> > +++ b/drivers/md/dm-crypt.c
> > @@ -75,8 +75,10 @@ struct dm_crypt_io {
> >   	struct bio *base_bio;
> >   	u8 *integrity_metadata;
> >   	bool integrity_metadata_from_pool:1;
> > +	bool in_tasklet:1;
> >   
> >   	struct work_struct work;
> > +	struct tasklet_struct tasklet;
> >   
> >   	struct convert_context ctx;
> >   
> > @@ -1775,6 +1777,7 @@ static void crypt_io_init(struct dm_crypt_io *io, struct crypt_config *cc,
> >   	io->ctx.r.req = NULL;
> >   	io->integrity_metadata = NULL;
> >   	io->integrity_metadata_from_pool = false;
> > +	io->in_tasklet = false;
> >   	atomic_set(&io->io_pending, 0);
> >   }
> >   
> > @@ -1785,6 +1788,13 @@ static void crypt_inc_pending(struct dm_crypt_io *io)
> >   
> >   static void kcryptd_queue_read(struct dm_crypt_io *io);
> >   
> > +static void kcryptd_io_bio_endio(struct work_struct *work)
> > +{
> > +	struct dm_crypt_io *io = container_of(work, struct dm_crypt_io, work);
> > +
> > +	bio_endio(io->base_bio);
> > +}
> > +
> >   /*
> >    * One of the bios was finished. Check for completion of
> >    * the whole request and correctly clean up the buffer.
> > @@ -1817,6 +1827,20 @@ static void crypt_dec_pending(struct dm_crypt_io *io)
> >   
> >   	base_bio->bi_status = error;
> >   
> > +	/*
> > +	 * If we are running this function from our tasklet,
> > +	 * we can't call bio_endio() here, because it will call
> > +	 * clone_endio() from dm.c, which in turn will
> > +	 * free the current struct dm_crypt_io structure with
> > +	 * our tasklet. In this case we need to delay bio_endio()
> > +	 * execution to after the tasklet is done and dequeued.
> > +	 */
> > +	if (io->in_tasklet) {
> > +		INIT_WORK(&io->work, kcryptd_io_bio_endio);
> > +		queue_work(cc->io_queue, &io->work);
> > +		return;
> > +	}
> > +
> >   	bio_endio(base_bio);
> >   }
> >   
> > @@ -2291,6 +2315,11 @@ static void kcryptd_crypt(struct work_struct *work)
> >   		kcryptd_crypt_write_convert(io);
> >   }
> >   
> > +static void kcryptd_crypt_tasklet(unsigned long work)
> > +{
> > +	kcryptd_crypt((struct work_struct *)work);
> > +}
> > +
> >   static void kcryptd_queue_crypt(struct dm_crypt_io *io)
> >   {
> >   	struct crypt_config *cc = io->cc;
> > @@ -2302,10 +2331,15 @@ static void kcryptd_queue_crypt(struct dm_crypt_io *io)
> >   		 * irqs_disabled(): the kernel may run some IO completion from the idle thread, but
> >   		 * it is being executed with irqs disabled.
> >   		 */
> > -		if (!(in_hardirq() || irqs_disabled())) {
> > -			kcryptd_crypt(&io->work);
> > +		if (in_hardirq() || irqs_disabled()) {
> > +			io->in_tasklet = true;
> > +			tasklet_init(&io->tasklet, kcryptd_crypt_tasklet, (unsigned long)&io->work);
> > +			tasklet_schedule(&io->tasklet);
> >   			return;
> >   		}
> > +
> > +		kcryptd_crypt(&io->work);
> > +		return;
> >   	}
> >   
> >   	INIT_WORK(&io->work, kcryptd_crypt);
> > diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> > index 49e4a35d7019..0bb126eadc0d 100644
> > --- a/drivers/md/dm-verity-target.c
> > +++ b/drivers/md/dm-verity-target.c
> > @@ -701,6 +701,23 @@ static void verity_work(struct work_struct *w)
> >   	verity_finish_io(io, errno_to_blk_status(verity_verify_io(io)));
> >   }
> >   
> > +static void verity_tasklet(unsigned long data)
> > +{
> > +	struct dm_verity_io *io = (struct dm_verity_io *)data;
> > +	int err;
> > +
> > +	io->in_tasklet = true;
> > +	err = verity_verify_io(io);
> > +	if (err == -EAGAIN || err == -ENOMEM) {
> > +		/* fallback to retrying with work-queue */
> > +		INIT_WORK(&io->work, verity_work);
> > +		queue_work(io->v->verify_wq, &io->work);
> > +		return;
> > +	}
> > +
> > +	verity_finish_io(io, errno_to_blk_status(err));
> > +}
> > +
> >   static void verity_end_io(struct bio *bio)
> >   {
> >   	struct dm_verity_io *io = bio->bi_private;
> > @@ -713,8 +730,13 @@ static void verity_end_io(struct bio *bio)
> >   		return;
> >   	}
> >   
> > -	INIT_WORK(&io->work, verity_work);
> > -	queue_work(io->v->verify_wq, &io->work);
> > +	if (static_branch_unlikely(&use_tasklet_enabled) && io->v->use_tasklet) {
> > +		tasklet_init(&io->tasklet, verity_tasklet, (unsigned long)io);
> > +		tasklet_schedule(&io->tasklet);
> > +	} else {
> > +		INIT_WORK(&io->work, verity_work);
> > +		queue_work(io->v->verify_wq, &io->work);
> > +	}
> >   }
> >   
> >   /*
> > diff --git a/drivers/md/dm-verity.h b/drivers/md/dm-verity.h
> > index db93a91169d5..7e495cc375b0 100644
> > --- a/drivers/md/dm-verity.h
> > +++ b/drivers/md/dm-verity.h
> > @@ -87,6 +87,7 @@ struct dm_verity_io {
> >   	bool in_tasklet;
> >   
> >   	struct work_struct work;
> > +	struct tasklet_struct tasklet;
> >   
> >   	char *recheck_buffer;
> >   
> > --
> > 2.31.1