[PATCH] sparc32: sun4m: do not clear IPIs that were not handled

[Ramin Moussavi]

maybe_smp4m_msg reads the per-cpu interrupt pending register and then
writes 0x70000000 to the clear register, acknowledging all three soft
interrupt levels used for IPIs (12, 13 and 14) whether or not they were
pending in the value it read. An IPI that another CPU raises between the
read and the write is acknowledged without ever being handled.

For IRQ_IPI_SINGLE this is not just a delay. __smp_call_single_queue()
only sends the IPI when the target's call_single_queue was empty, so
once one IPI is lost no further IPI is sent to that CPU, and every
remote wakeup queued there waits until the CPU next goes idle and
flushes the queue. Under CPU-bound load that does not happen: woken
tasks show up as R in /proc, with a vruntime below that of the tasks
that keep running, and still do not run until the load ends.

Seen on a 4-CPU SPARCstation 20 under qemu with eight threads doing
syscalls and window flushes next to threads that wake each other
through condition variables and pipes: 7 of 8 runs had tasks starving
for as long as the load lasted (up to 25 minutes), 0 of 4 runs with a
single CPU. With this change 0 of 8 runs starved.

Acknowledge only the levels that were seen. A level raised after the
read stays pending and traps again once PIL allows.

Fixes: ecbc42b70acb ("sparc32, sun4m: Implemented SMP IPIs support for SUN4M machines")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Ramin Moussavi <ramin.moussavi@xxxxxxxxx>
---
 arch/sparc/kernel/entry.S | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/sparc/kernel/entry.S b/arch/sparc/kernel/entry.S
index ea51ef52c952..0bc30436f028 100644
--- a/arch/sparc/kernel/entry.S
+++ b/arch/sparc/kernel/entry.S
@@ -275,11 +275,11 @@ maybe_smp4m_msg:
 	sethi	%hi(0x70000000), %o2	! Check all soft-IRQs
 	ld	[%o5 + %o3], %o1
 	ld	[%o1 + 0x00], %o3	! sun4m_irq_percpu[cpu]->pending
-	andcc	%o3, %o2, %g0
+	andcc	%o3, %o2, %o4
 	be,a	smp4m_ticker
 	 cmp	%l7, 14
-	/* Soft-IRQ IPI */
-	st	%o2, [%o1 + 0x04]	! sun4m_irq_percpu[cpu]->clear=0x70000000
+	/* Soft-IRQ IPI: clear only the ones we saw, a later one must stay */
+	st	%o4, [%o1 + 0x04]	! sun4m_irq_percpu[cpu]->clear
 	WRITE_PAUSE
 	ld	[%o1 + 0x00], %g0	! sun4m_irq_percpu[cpu]->pending
 	WRITE_PAUSE
-- 
2.53.0