Mark HI and TASKLET softirq synchronous

Way back in 4.9, we committed 4cd13c21b207 ("softirq: Let ksoftirqd do
its job"), and ever since we've had small nagging issues with it.  For
example, we've had:

  1ff688209e2e ("watchdog: core: make sure the watchdog_worker is not deferred")
  8d5755b3f77b ("watchdog: softdog: fire watchdog even if softirqs do not get to run")
  217f69743681 ("net: busy-poll: allow preemption in sk_busy_loop()")

all of which worked around some of the effects of that commit.

The DVB people have also complained that the commit causes excessive USB
URB latencies, which seems to be due to the USB code using tasklets to
schedule USB traffic.  This seems to be an issue mainly when already
living on the edge, but waiting for ksoftirqd to handle it really does
seem to cause excessive latencies.

Now Hanna Hawa reports that this issue isn't just limited to USB URB and
DVB, but also causes timeout problems for the Marvell SoC team:

 "I'm facing kernel panic issue while running raid 5 on sata disks
  connected to Macchiatobin (Marvell community board with Armada-8040
  SoC with 4 ARMv8 cores of CA72) Raid 5 built with Marvell DMA engine
  and async_tx mechanism (ASYNC_TX_DMA [=y]); the DMA driver (mv_xor_v2)
  uses a tasklet to clean the done descriptors from the queue"

The latency problem causes a panic:

  mv_xor_v2 f0400000.xor: dma_sync_wait: timeout!
  Kernel panic - not syncing: async_tx_quiesce: DMA error waiting for transaction

We've discussed simply just reverting the original commit entirely, and
also much more involved solutions (with per-softirq threads etc).  This
patch is intentionally stupid and fairly limited, because the issue
still remains, and the other solutions either got sidetracked or had
other issues.

We should probably also consider the timer softirqs to be synchronous
and not be delayed to ksoftirqd (since they were the issue with the
earlier watchdog problems), but that should be done as a separate patch.
This does only the tasklet cases.

Reported-and-tested-by: Hanna Hawa <hannah@marvell.com>
Reported-and-tested-by: Josef Griebichler <griebichler.josef@gmx.at>
Reported-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/softirq.c

@@ -79,12 +79,16 @@ static void wakeup_softirqd(void)
 
 /*
  * If ksoftirqd is scheduled, we do not want to process pending softirqs
- * right now. Let ksoftirqd handle this at its own rate, to get fairness.
+ * right now. Let ksoftirqd handle this at its own rate, to get fairness,
+ * unless we're doing some of the synchronous softirqs.
  */
-static bool ksoftirqd_running(void)
+#define SOFTIRQ_NOW_MASK ((1 << HI_SOFTIRQ) | (1 << TASKLET_SOFTIRQ))
+static bool ksoftirqd_running(unsigned long pending)
 {
 	struct task_struct *tsk = __this_cpu_read(ksoftirqd);
 
+	if (pending & SOFTIRQ_NOW_MASK)
+		return false;
 	return tsk && (tsk->state == TASK_RUNNING);
 }
 
@@ -328,7 +332,7 @@ asmlinkage __visible void do_softirq(void)
 
 	pending = local_softirq_pending();
 
-	if (pending && !ksoftirqd_running())
+	if (pending && !ksoftirqd_running(pending))
 		do_softirq_own_stack();
 
 	local_irq_restore(flags);
@@ -355,7 +359,7 @@ void irq_enter(void)
 
 static inline void invoke_softirq(void)
 {
-	if (ksoftirqd_running())
+	if (ksoftirqd_running(local_softirq_pending()))
 		return;
 
 	if (!force_irqthreads) {