[uml-devel] [PATCH 1/2] Prevent IRQ handler reentrancy

[uml-devel] [PATCH 1/2] Prevent IRQ handler reentrancy

[Anton Ivanov]

The existing IRQ handler design in UML does not prevent reentrancy

This is mitigated by fd-enable/fd-disable semantics for the IO
portion of the UML subsystem. The timer, however, can and is
re-entered resulting in very deep stack usage and occasional
stack exhaustion.

This patch prevents this by checking if there is a timer
interrupt in-flight before processing any pending timer interrupts.

Signed-off-by: Anton Ivanov <aivanov@brocade.com>
---
 arch/um/os-Linux/signal.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/um/os-Linux/signal.c b/arch/um/os-Linux/signal.c
index c211153..7801666 100644
--- a/arch/um/os-Linux/signal.c
+++ b/arch/um/os-Linux/signal.c
@@ -62,6 +62,7 @@ static void sig_handler_common(int sig, struct siginfo *si, mcontext_t *mc)
 
 static int signals_enabled;
 static unsigned int signals_pending;
+static unsigned int signals_active = 0;
 
 void sig_handler(int sig, struct siginfo *si, mcontext_t *mc)
 {
@@ -101,7 +102,12 @@ void timer_alarm_handler(int sig, struct siginfo *unused_si, mcontext_t *mc)
 
 	block_signals();
 
+	signals_active |= SIGALRM_MASK;
+
 	timer_real_alarm_handler(mc);
+
+	signals_active &= ~SIGALRM_MASK;
+
 	set_signals(enabled);
 }
 
@@ -286,8 +292,16 @@ void unblock_signals(void)
 		if (save_pending & SIGIO_MASK)
 			sig_handler_common(SIGIO, NULL, NULL);
 
-		if (save_pending & SIGALRM_MASK)
+		/* Do not reenter the handler */
+
+		if ((save_pending & SIGALRM_MASK) && (!(signals_active & SIGALRM_MASK)))
 			timer_real_alarm_handler(NULL);
+
+		/* Rerun the loop only if there is still pending SIGIO and not in TIMER handler */
+
+		if (!(signals_pending & SIGIO_MASK) && (signals_active & SIGALRM_MASK))
+			return;
+
 	}
 }
 
-- 
2.1.4


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[uml-devel] [PATCH 2/2] Optimization - do not change hard IRQ flags in soft IRQ processing

[Anton Ivanov]

Software IRQ processing in generic architectures assumes that the
exit out of hard IRQ may have re-enabled interrupts (some
architectures may have an implicit EOI). It presumes them enabled
and toggles the flags once more just in case unless this is turned
off in the architecture specific hardirq.h by setting
__ARCH_IRQ_EXIT_IRQS_DISABLED

This patch adds this to UML where due to the way IRQs are handled
it is an optimization (it works fine without it too).

Signed-off-by: Anton Ivanov <aivanov@brocade.com>
---
 arch/um/include/asm/hardirq.h | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)
 create mode 100644 arch/um/include/asm/hardirq.h

diff --git a/arch/um/include/asm/hardirq.h b/arch/um/include/asm/hardirq.h
new file mode 100644
index 0000000..756f077
--- /dev/null
+++ b/arch/um/include/asm/hardirq.h
@@ -0,0 +1,23 @@
+#ifndef __ASM_UM_HARDIRQ_H
+#define __ASM_UM_HARDIRQ_H
+
+#include <linux/cache.h>
+#include <linux/threads.h>
+
+typedef struct {
+	unsigned int __softirq_pending;
+} ____cacheline_aligned irq_cpustat_t;
+
+#include <linux/irq_cpustat.h>	/* Standard mappings for irq_cpustat_t above */
+#include <linux/irq.h>
+
+#ifndef ack_bad_irq
+static inline void ack_bad_irq(unsigned int irq)
+{
+	printk(KERN_CRIT "unexpected IRQ trap at vector %02x\n", irq);
+}
+#endif
+
+#define __ARCH_IRQ_EXIT_IRQS_DISABLED 1
+
+#endif /* __ASM_UM_HARDIRQ_H */
-- 
2.1.4


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