drivers: complete shared-IRQ re-arm sweep (audio, net, graphics)

Continuation of 92924224 — the same conditional-ack bug class found in
six more drivers: a foreign interrupt on a shared INTx line skipped the
write-back ack, leaving the kernel-masked IRQ line dead forever.

- sb16d, ac97d: ack before the not-ours continue
- rtl8139d, rtl8168d, ixgbed: ack unconditionally, tick only when ours
  (the stale 'TODO: spurious interrupts' comments removed — the
  spurious-interrupt confusion was this bug)
- ihdgd: ack unconditionally, handle_events/tick only when ours

Verified: cargo check -Z build-std --target x86_64-unknown-redox for
sb16d, ac97d, rtl8139d, rtl8168d, ixgbed, ihdgd — clean, no new
warnings. nvmed audited: not affected (per-queue MSI pattern, no
conditional ack).

drivers/audio/ac97d/src/main.rs

@@ -149,14 +149,19 @@ fn daemon(daemon: daemon::Daemon, pcid_handle: PciFunctionHandle) -> ! {
                     std::process::exit(1);
                 }
 
-                if !device.irq() {
-                    continue;
-                }
+                let ours = device.irq();
+                // The kernel masks the IRQ line on delivery and only re-arms
+                // it on write-back; ack unconditionally or a foreign interrupt
+                // on a shared line leaves the line masked forever.
                 if let Err(err) = irq_file.write(&mut irq) {
                     error!("ac97d: failed to acknowledge IRQ: {err}");
                     std::process::exit(1);
                 }
 
+                if !ours {
+                    continue;
+                }
+
                 readiness_based
                     .poll_all_requests(&mut device)
                     .unwrap_or_else(|err| {

drivers/audio/sb16d/src/main.rs

@@ -79,10 +79,15 @@ fn daemon(daemon: daemon::Daemon) -> ! {
                 let mut irq = [0; 8];
                 irq_file.read(&mut irq).unwrap();
 
-                if !device.irq() {
+                let ours = device.irq();
+                // The kernel masks the IRQ line on delivery and only re-arms
+                // it on write-back; ack unconditionally or a foreign interrupt
+                // on a shared line leaves the line masked forever.
+                irq_file.write(&mut irq).unwrap();
+
+                if !ours {
                     continue;
                 }
-                irq_file.write(&mut irq).unwrap();
 
                 readiness_based
                     .poll_all_requests(&mut device)

drivers/graphics/ihdgd/src/main.rs

@@ -86,9 +86,13 @@ fn daemon(daemon: daemon::Daemon, mut pcid_handle: PciFunctionHandle) -> ! {
             Source::Irq => {
                 let mut irq = [0; 8];
                 irq_file.irq_handle().read(&mut irq).unwrap();
-                if scheme.adapter_mut().handle_irq() {
-                    irq_file.irq_handle().write(&mut irq).unwrap();
+                let ours = scheme.adapter_mut().handle_irq();
+                // The kernel masks the IRQ line on delivery and only re-arms
+                // it on write-back; ack unconditionally or a foreign interrupt
+                // on a shared line leaves the line masked forever.
+                irq_file.irq_handle().write(&mut irq).unwrap();
 
+                if ours {
                     scheme.adapter_mut().handle_events();
                     scheme.tick().unwrap();
                 }

drivers/net/ixgbed/src/main.rs

@@ -73,9 +73,13 @@ fn daemon(daemon: daemon::Daemon, mut pcid_handle: PciFunctionHandle) -> ! {
             Source::Irq => {
                 let mut irq = [0; 8];
                 irq_file.read(&mut irq).unwrap();
-                if scheme.adapter().irq() {
-                    irq_file.write(&mut irq).unwrap();
+                let ours = scheme.adapter().irq();
+                // The kernel masks the IRQ line on delivery and only re-arms
+                // it on write-back; ack unconditionally or a foreign interrupt
+                // on a shared line leaves the line masked forever.
+                irq_file.write(&mut irq).unwrap();
 
+                if ours {
                     scheme.tick().unwrap();
                 }
             }

drivers/net/rtl8139d/src/main.rs

@@ -99,10 +99,13 @@ fn daemon(daemon: daemon::Daemon, mut pcid_handle: PciFunctionHandle) -> ! {
             Source::Irq => {
                 let mut irq = [0; 8];
                 irq_file.irq_handle().read(&mut irq).unwrap();
-                //TODO: This may be causing spurious interrupts
-                if unsafe { scheme.adapter_mut().irq() } {
-                    irq_file.irq_handle().write(&mut irq).unwrap();
+                let ours = unsafe { scheme.adapter_mut().irq() };
+                // The kernel masks the IRQ line on delivery and only re-arms
+                // it on write-back; ack unconditionally or a foreign interrupt
+                // on a shared line leaves the line masked forever.
+                irq_file.irq_handle().write(&mut irq).unwrap();
 
+                if ours {
                     scheme.tick().unwrap();
                 }
             }

drivers/net/rtl8168d/src/main.rs

@@ -99,10 +99,13 @@ fn daemon(daemon: daemon::Daemon, mut pcid_handle: PciFunctionHandle) -> ! {
             Source::Irq => {
                 let mut irq = [0; 8];
                 irq_file.irq_handle().read(&mut irq).unwrap();
-                //TODO: This may be causing spurious interrupts
-                if unsafe { scheme.adapter_mut().irq() } {
-                    irq_file.irq_handle().write(&mut irq).unwrap();
+                let ours = unsafe { scheme.adapter_mut().irq() };
+                // The kernel masks the IRQ line on delivery and only re-arms
+                // it on write-back; ack unconditionally or a foreign interrupt
+                // on a shared line leaves the line masked forever.
+                irq_file.irq_handle().write(&mut irq).unwrap();
 
+                if ours {
                     scheme.tick().unwrap();
                 }
             }