An announcement for kernel-global workqueue users.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Hello.

The Linux kernel provides kernel-global WQs (namely, system_wq, system_highpri_wq,
system_long_wq, system_unbound_wq, system_freezable_wq, system_power_efficient_wq
and system_freezable_power_efficient_wq). But since attempt to flush kernel-global
WQs has possibility of deadlock, Tejun Heo thinks that we should stop calling
flush_scheduled_work() and flush_workqueue(system_*). Such callers as of Linux 5.17
are listed below.

----------
$ git grep -nF 'flush_scheduled_work()'
drivers/acpi/osl.c:1182:         * invoke flush_scheduled_work()/acpi_os_wait_events_complete() to flush
drivers/acpi/osl.c:1575:        flush_scheduled_work();
drivers/block/aoe/aoedev.c:324: flush_scheduled_work();
drivers/block/aoe/aoedev.c:523: flush_scheduled_work();
drivers/crypto/atmel-ecc.c:401: flush_scheduled_work();
drivers/crypto/atmel-sha204a.c:162:     flush_scheduled_work();
drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c:2606:       flush_scheduled_work();
drivers/gpu/drm/bridge/lontium-lt9611uxc.c:985: flush_scheduled_work();
drivers/gpu/drm/i915/display/intel_display.c:10790:     flush_scheduled_work();
drivers/gpu/drm/i915/gt/selftest_execlists.c:87:        flush_scheduled_work();
drivers/iio/light/tsl2563.c:811:        flush_scheduled_work();
drivers/infiniband/hw/mlx4/cm.c:511:            flush_scheduled_work();
drivers/infiniband/hw/mlx4/cm.c:543:            flush_scheduled_work(); /* make sure all timers were flushed */
drivers/infiniband/ulp/isert/ib_isert.c:2639:   flush_scheduled_work();
drivers/input/mouse/psmouse-smbus.c:320:        flush_scheduled_work();
drivers/md/dm.c:229:    flush_scheduled_work();
drivers/message/fusion/mptscsih.c:1234: flush_scheduled_work();
drivers/net/phy/phy.c:1060:     /* Cannot call flush_scheduled_work() here as desired because
drivers/net/usb/lan78xx.c:3240:  * can't flush_scheduled_work() until we drop rtnl (later),
drivers/net/usb/usbnet.c:853:    * can't flush_scheduled_work() until we drop rtnl (later),
drivers/net/wireless/ath/ath6kl/usb.c:481:      flush_scheduled_work();
drivers/net/wwan/wwan_hwsim.c:537:      flush_scheduled_work();         /* Wait deletion works completion */
drivers/nvme/target/configfs.c:1557:    flush_scheduled_work();
drivers/nvme/target/rdma.c:1587:                flush_scheduled_work();
drivers/nvme/target/rdma.c:2056:        flush_scheduled_work();
drivers/nvme/target/tcp.c:1818:         flush_scheduled_work();
drivers/nvme/target/tcp.c:1879: flush_scheduled_work();
drivers/nvme/target/tcp.c:1884: flush_scheduled_work();
drivers/platform/surface/surface_acpi_notify.c:863:     flush_scheduled_work();
drivers/power/supply/ab8500_btemp.c:975:        flush_scheduled_work();
drivers/power/supply/ab8500_chargalg.c:1993:    flush_scheduled_work();
drivers/power/supply/ab8500_charger.c:3400:     flush_scheduled_work();
drivers/power/supply/ab8500_fg.c:3021:  flush_scheduled_work();
drivers/rapidio/devices/tsi721.c:2944:  flush_scheduled_work();
drivers/rtc/dev.c:99:                   flush_scheduled_work();
drivers/scsi/mpt3sas/mpt3sas_scsih.c:12409:     flush_scheduled_work();
drivers/scsi/qla2xxx/qla_target.c:1568:         flush_scheduled_work();
drivers/staging/olpc_dcon/olpc_dcon.c:386:      flush_scheduled_work();
sound/soc/intel/atom/sst/sst.c:363:     flush_scheduled_work();
$ git grep -nF 'flush_workqueue(system_'
drivers/block/rnbd/rnbd-clt.c:1776:     flush_workqueue(system_long_wq);
drivers/infiniband/core/device.c:2857:  flush_workqueue(system_unbound_wq);
include/linux/workqueue.h:592:  flush_workqueue(system_wq);
----------

I tried to send a patch that emits a warning when flushing kernel-global WQs is attempted
( https://lkml.kernel.org/r/2efd5461-fccd-f1d9-7138-0a6767cbf5fe@I-love.SAKURA.ne.jp ).
But Linus does not want such patch
( https://lkml.kernel.org/r/CAHk-=whWreGjEQ6yasspzBrNnS7EQiL+SknToWt=SzUh4XomyQ@mail.gmail.com ).

Steps for converting kernel-global WQs into module's local WQs are shown below.
But since an oversight in Step 4 results in breakage, I think that this conversion
should be carefully handled by maintainers/developers of each module who are
familiar with that module. (This is why I'm sending this mail than sending patches,
in order to ask for your cooperation.)

----------
Step 0: Consider if flushing kernel-global WQs is unavoidable.

    For example, commit 081bdc9fe05bb232 ("RDMA/ib_srp: Fix a deadlock")
    simply removed flush_workqueue(system_long_wq) call.

    For another example, schedule_on_each_cpu() does not need to call
    flush_scheduled_work() because schedule_on_each_cpu() knows the list
    of all "struct work_struct" instances which need to be flushed using
    flush_work() call.

    If flushing kernel-global WQs is still unavoidable, please proceed to
    the following steps.

Step 1: Declare a variable for your module.

    struct workqueue_struct *my_wq;

Step 2: Create a WQ for your module from __init function. The same flags
        used by corresponding kernel-global WQ can be used when creating
        the WQ for your module.

    my_wq = alloc_workqueue("my_wq_name", 0, 0);

Step 3: Destroy the WQ created in Step 2 from __exit function (and the error
        handling path of __init function if __init function may fail after
        creating the WQ).

    destroy_workqueue(my_wq);

Step 4: Replace e.g. schedule_work() call with corresponding queue_work() call
        throughout your module which should be handled by the WQ for your module.

Step 5: Replace flush_scheduled_work() and flush_workqueue(system_*) calls
        with flush_workqueue() of the WQ for your module.

    flush_workqueue(my_wq);
----------

Regards.