linux-kernel - [RFC PATCH v2 0/2] ceph_check_delayed

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Message-Id: <20210629094749.25253-1-lhenriques@suse.de>
Date:   Tue, 29 Jun 2021 10:47:47 +0100
From:   Luis Henriques <lhenriques@...e.de>
To:     Jeff Layton <jlayton@...nel.org>, Ilya Dryomov <idryomov@...il.com>
Cc:     ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Luis Henriques <lhenriques@...e.de>
Subject: [RFC PATCH v2 0/2] ceph_check_delayed_caps() softlockup

This is an attempt to fix the softlock on the delayed_work workqueue.  As
stated in 0002 patch:

  Function ceph_check_delayed_caps() is called from the mdsc->delayed_work
  workqueue and it can be kept looping for quite some time if caps keep being
  added back to the mdsc->cap_delay_list.  This may result in the watchdog
  tainting the kernel with the softlockup flag.

v2 of this fix modifies the approach by time-bounding the loop in this
function, so that any caps added to the list *after* the loop starts will
be postponed to the next wq run.

An extra change in 0001 (suggested by Jeff) allows scheduling runs for
periods smaller than the default (5 secs) period.  This way,
delayed_work() can have the next run scheduled for the next list element
ci->i_hold_caps_max instead of 5 secs.

This patchset should fix the issue reported here [1], although a quick
search for "ceph_check_delayed_caps" in the tracker returns a few more
bugs, possibly duplicates.

[1] https://tracker.ceph.com/issues/46284

Luis Henriques (2):
  ceph: allow schedule_delayed() callers to set delay for workqueue
  ceph: reduce contention in ceph_check_delayed_caps()

 fs/ceph/caps.c       | 17 ++++++++++++++++-
 fs/ceph/mds_client.c | 24 +++++++++++++++---------
 fs/ceph/super.h      |  2 +-
 3 files changed, 32 insertions(+), 11 deletions(-)