Messages in this thread | | | Date | Thu, 20 Feb 2014 20:44:46 -0500 | From | Peter Hurley <> | Subject | Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED_WORK |
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On 02/20/2014 03:44 PM, Tejun Heo wrote: > PREPARE_[DELAYED_]WORK() are being phased out. They have few users > and a nasty surprise in terms of reentrancy guarantee as workqueue > considers work items to be different if they don't have the same work > function. > > firewire core-device and sbp2 have been been multiplexing work items > with multiple work functions. Introduce fw_device_workfn() and > sbp2_lu_workfn() which invoke fw_device->workfn and > sbp2_logical_unit->workfn respectively and always use the two > functions as the work functions and update the users to set the > ->workfn fields instead of overriding work functions using > PREPARE_DELAYED_WORK(). > > It would probably be best to route this with other related updates > through the workqueue tree. > > Compile tested. > > Signed-off-by: Tejun Heo <tj@kernel.org> > Cc: Stefan Richter <stefanr@s5r6.in-berlin.de> > Cc: linux1394-devel@lists.sourceforge.net > Cc: Chris Boot <bootc@bootc.net> > Cc: linux-scsi@vger.kernel.org > Cc: target-devel@vger.kernel.org > --- > drivers/firewire/core-device.c | 22 +++++++++++++++------- > drivers/firewire/sbp2.c | 17 +++++++++++++---- > include/linux/firewire.h | 1 + > 3 files changed, 29 insertions(+), 11 deletions(-) > > diff --git a/drivers/firewire/core-device.c b/drivers/firewire/core-device.c > index de4aa40..2c6d5e1 100644 > --- a/drivers/firewire/core-device.c > +++ b/drivers/firewire/core-device.c > @@ -916,7 +916,7 @@ static int lookup_existing_device(struct device *dev, void *data) > old->config_rom_retries = 0; > fw_notice(card, "rediscovered device %s\n", dev_name(dev)); > > - PREPARE_DELAYED_WORK(&old->work, fw_device_update); > + old->workfn = fw_device_update; > fw_schedule_device_work(old, 0); > > if (current_node == card->root_node) > @@ -1075,7 +1075,7 @@ static void fw_device_init(struct work_struct *work) > if (atomic_cmpxchg(&device->state, > FW_DEVICE_INITIALIZING, > FW_DEVICE_RUNNING) == FW_DEVICE_GONE) { > - PREPARE_DELAYED_WORK(&device->work, fw_device_shutdown); > + device->workfn = fw_device_shutdown; > fw_schedule_device_work(device, SHUTDOWN_DELAY);
Implied mb of test_and_set_bit() in queue_work_on() ensures that the newly assigned work function is visible on all cpus before evaluating whether or not the work can be queued.
Ok.
> } else { > fw_notice(card, "created device %s: GUID %08x%08x, S%d00\n", > @@ -1196,13 +1196,20 @@ static void fw_device_refresh(struct work_struct *work) > dev_name(&device->device), fw_rcode_string(ret)); > gone: > atomic_set(&device->state, FW_DEVICE_GONE); > - PREPARE_DELAYED_WORK(&device->work, fw_device_shutdown); > + device->workfn = fw_device_shutdown; > fw_schedule_device_work(device, SHUTDOWN_DELAY); > out: > if (node_id == card->root_node->node_id) > fw_schedule_bm_work(card, 0); > } > > +static void fw_device_workfn(struct work_struct *work) > +{ > + struct fw_device *device = container_of(to_delayed_work(work), > + struct fw_device, work);
I think this needs an smp_rmb() here.
> + device->workfn(work); > +} > +
Otherwise this cpu could speculatively load workfn before set_work_pool_and_clear_pending(), which means that the old workfn could have been loaded but PENDING was still set and caused queue_work_on() to reject the work as already pending.
Result: the new work function never runs.
But this exposes a more general problem that I believe workqueue should prevent; speculated loads and stores in the work item function should be prevented from occurring before clearing PENDING in set_work_pool_and_clear_pending().
IOW, the beginning of the work function should act like a barrier in the same way that queue_work_on() (et. al.) already does.
Regards, Peter Hurley
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