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    SubjectRe: [PATCH 17/22] cifs: Use alloc_ordered_workqueue() to create ordered workqueues
    Date
    Tejun Heo <tj@kernel.org> writes:

    > BACKGROUND
    > ==========
    >
    > When multiple work items are queued to a workqueue, their execution order
    > doesn't match the queueing order. They may get executed in any order and
    > simultaneously. When fully serialized execution - one by one in the queueing
    > order - is needed, an ordered workqueue should be used which can be created
    > with alloc_ordered_workqueue().
    >
    > However, alloc_ordered_workqueue() was a later addition. Before it, an
    > ordered workqueue could be obtained by creating an UNBOUND workqueue with
    > @max_active==1. This originally was an implementation side-effect which was
    > broken by 4c16bd327c74 ("workqueue: restore WQ_UNBOUND/max_active==1 to be
    > ordered"). Because there were users that depended on the ordered execution,
    > 5c0338c68706 ("workqueue: restore WQ_UNBOUND/max_active==1 to be ordered")
    > made workqueue allocation path to implicitly promote UNBOUND workqueues w/
    > @max_active==1 to ordered workqueues.
    >
    > While this has worked okay, overloading the UNBOUND allocation interface
    > this way creates other issues. It's difficult to tell whether a given
    > workqueue actually needs to be ordered and users that legitimately want a
    > min concurrency level wq unexpectedly gets an ordered one instead. With
    > planned UNBOUND workqueue updates to improve execution locality and more
    > prevalence of chiplet designs which can benefit from such improvements, this
    > isn't a state we wanna be in forever.
    >
    > This patch series audits all callsites that create an UNBOUND workqueue w/
    > @max_active==1 and converts them to alloc_ordered_workqueue() as necessary.
    >
    > WHAT TO LOOK FOR
    > ================
    >
    > The conversions are from
    >
    > alloc_workqueue(WQ_UNBOUND | flags, 1, args..)
    >
    > to
    >
    > alloc_ordered_workqueue(flags, args...)
    >
    > which don't cause any functional changes. If you know that fully ordered
    > execution is not ncessary, please let me know. I'll drop the conversion and
    > instead add a comment noting the fact to reduce confusion while conversion
    > is in progress.
    >
    > If you aren't fully sure, it's completely fine to let the conversion
    > through. The behavior will stay exactly the same and we can always
    > reconsider later.
    >
    > As there are follow-up workqueue core changes, I'd really appreciate if the
    > patch can be routed through the workqueue tree w/ your acks. Thanks.
    >
    > Signed-off-by: Tejun Heo <tj@kernel.org>
    > Cc: Steve French <sfrench@samba.org>
    > Cc: Paulo Alcantara <pc@cjr.nz>
    > Cc: Ronnie Sahlberg <lsahlber@redhat.com>
    > Cc: Shyam Prasad N <sprasad@microsoft.com>
    > Cc: Tom Talpey <tom@talpey.com>
    > Cc: linux-cifs@vger.kernel.org
    > Cc: samba-technical@lists.samba.org
    > ---
    > fs/cifs/dfs_cache.c | 2 +-
    > 1 file changed, 1 insertion(+), 1 deletion(-)

    Acked-by: Paulo Alcantara (SUSE) <pc@manguebit.com>

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