linux-sg2042/include
Thomas Gleixner a5c6234e10 completion: Use simple wait queues
completion uses a wait_queue_head_t to enqueue waiters.

wait_queue_head_t contains a spinlock_t to protect the list of waiters
which excludes it from being used in truly atomic context on a PREEMPT_RT
enabled kernel.

The spinlock in the wait queue head cannot be replaced by a raw_spinlock
because:

  - wait queues can have custom wakeup callbacks, which acquire other
    spinlock_t locks and have potentially long execution times

  - wake_up() walks an unbounded number of list entries during the wake up
    and may wake an unbounded number of waiters.

For simplicity and performance reasons complete() should be usable on
PREEMPT_RT enabled kernels.

completions do not use custom wakeup callbacks and are usually single
waiter, except for a few corner cases.

Replace the wait queue in the completion with a simple wait queue (swait),
which uses a raw_spinlock_t for protecting the waiter list and therefore is
safe to use inside truly atomic regions on PREEMPT_RT.

There is no semantical or functional change:

  - completions use the exclusive wait mode which is what swait provides

  - complete() wakes one exclusive waiter

  - complete_all() wakes all waiters while holding the lock which protects
    the wait queue against newly incoming waiters. The conversion to swait
    preserves this behaviour.

complete_all() might cause unbound latencies with a large number of waiters
being woken at once, but most complete_all() usage sites are either in
testing or initialization code or have only a really small number of
concurrent waiters which for now does not cause a latency problem. Keep it
simple for now.

The fixup of the warning check in the USB gadget driver is just a straight
forward conversion of the lockless waiter check from one waitqueue type to
the other.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/20200321113242.317954042@linutronix.de
2020-03-21 16:00:24 +01:00
..
acpi acpi: Remove header dependency 2020-03-21 16:00:21 +01:00
asm-generic asm-generic/bitops: Update stale comment 2020-03-06 11:06:19 +01:00
clocksource clocksource/drivers/hyper-v: Untangle stimers and timesync from clocksources 2020-01-16 19:09:02 +01:00
crypto crypto: poly1305 - add new 32 and 64-bit generic versions 2020-01-16 15:18:12 +08:00
drm drm/amdgpu: fix doc by clarifying sched_list definition 2020-01-27 16:46:44 -05:00
dt-bindings ARM: SoC: late updates 2020-02-08 14:17:27 -08:00
keys
kunit kunit: allow kunit tests to be loaded as a module 2020-01-09 16:42:29 -07:00
kvm
linux completion: Use simple wait queues 2020-03-21 16:00:24 +01:00
math-emu
media media: cec: remove unused functions 2020-01-08 14:38:06 +01:00
misc
net net: sched: correct flower port blocking 2020-02-17 21:33:28 -08:00
pcmcia
ras
rdma RDMA/core: Make the entire API tree static 2020-01-30 16:28:52 -04:00
scsi scsi: Revert "target: iscsi: Wait for all commands to finish before freeing a session" 2020-02-14 17:13:54 -05:00
soc ARM: SoC-related driver updates 2020-02-08 14:04:19 -08:00
sound ASoC: Fixes for v5.6 2020-02-18 13:33:39 +01:00
target
trace ARM: SoC-related driver updates 2020-02-08 14:04:19 -08:00
uapi USB fixes for 5.6-rc3 2020-02-21 12:44:53 -08:00
vdso lib/vdso: Prepare for time namespace support 2020-01-14 12:20:57 +01:00
video
xen xen: branch for v5.6-rc1 2020-02-05 17:44:14 +00:00