Re: [PATCH] padata: fix lockdep warning in padata serialization

[Daniel Jordan]

On Wed, Sep 21, 2022 at 09:36:16AM +0200, Steffen Klassert wrote:
> On Tue, Sep 20, 2022 at 10:10:57AM -0400, Daniel Jordan wrote:
> > Yeah, padata_do_serial can be called with BHs off, like in the tipc
> > stack, but there are also cases where BHs can be on, like lockdep said
> > here:
> 
> padata_do_serial was designed to run with BHs off, it is a bug if it
> runs with BHs on. But I don't see a case where this can happen. The
> only user of padata_do_serial is pcrypt in its serialization callbacks
> (pcrypt_aead_enc, pcrypt_aead_dec) and the async crypto callback
> pcrypt_aead_done. pcrypt_aead_enc and pcrypt_aead_dec are issued via
> the padata_serial_worker with the padata->serial call. BHs are
> off here. The crypto callback also runs with BHs off.
> 
> What do I miss here?

Ugh.. this newer, buggy part of padata_do_parallel:

  /* Maximum works limit exceeded, run in the current task. */
  padata->parallel(padata);

This skips the usual path in padata_parallel_worker, which disables BHs.
They should be left off in the above case too.

What about this?

---8<---

Subject: [PATCH] padata: always leave BHs disabled when running ->parallel()

A deadlock can happen when an overloaded system runs ->parallel() in the
context of the current task:

    padata_do_parallel
      ->parallel()
        pcrypt_aead_enc/dec
          padata_do_serial
            spin_lock(&reorder->lock) // BHs still enabled
              <interrupt>
                ...
                  __do_softirq
                    ...
                      padata_do_serial
                        spin_lock(&reorder->lock)

It's a bug for BHs to be on in _do_serial as Steffen points out, so
ensure they're off in the "current task" case like they are in
padata_parallel_worker to avoid this situation.

Reported-by: syzbot+bc05445bc14148d51915@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 4611ce224688 ("padata: allocate work structures for parallel jobs from a pool")
Signed-off-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
---
 kernel/padata.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/padata.c b/kernel/padata.c
index e5819bb8bd1d..97f51e0c1776 100644
--- a/kernel/padata.c
+++ b/kernel/padata.c
@@ -207,14 +207,16 @@ int padata_do_parallel(struct padata_shell *ps,
 	pw = padata_work_alloc();
 	spin_unlock(&padata_works_lock);
 
+	if (!pw) {
+		/* Maximum works limit exceeded, run in the current task. */
+		padata->parallel(padata);
+	}
+
 	rcu_read_unlock_bh();
 
 	if (pw) {
 		padata_work_init(pw, padata_parallel_worker, padata, 0);
 		queue_work(pinst->parallel_wq, &pw->pw_work);
-	} else {
-		/* Maximum works limit exceeded, run in the current task. */
-		padata->parallel(padata);
 	}
 
 	return 0;
-- 
2.37.2