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From:   Johannes Berg <johannes@sipsolutions.net>
To:     linux-um@lists.infradead.org
Cc:     linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@kernel.org>,
        Johannes Berg <johannes.berg@intel.com>
Subject: [PATCH 4/7] um: time-travel/signals: fix ndelay() in interrupt
Date:   Tue, 23 Feb 2021 16:27:04 +0100
Message-Id: <20210223161449.7461e0b6dacc.I3507b456d425cb95a092eb50b6cb491fe8575c50@changeid>
In-Reply-To: <20210223152707.408995-1-johannes@sipsolutions.net>
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From: Johannes Berg <johannes.berg@intel.com>

We should be able to ndelay() from any context, even from an
interrupt context! However, this is broken (not functionally,
but locking-wise) in time-travel because we'll get into the
time-travel code and enable interrupts to handle messages on
other time-travel aware subsystems (only virtio for now).

Luckily, I've already reworked the time-travel aware signal
(interrupt) delivery for suspend/resume to have a time travel
handler, which runs directly in the context of the signal and
not from the Linux interrupt.

In order to fix this time-travel issue then, we need to do a
few things:

 1) rework the signal handling code to not block SIGIO (if
    time-travel is enabled, just to simplify the other case)
    but rather let it bubble through the system, all the way
    *past* the IRQ's timetravel_handler, stopping it after
    that and before real interrupt delivery if IRQs are not
    actually enabled;
 2) rework time-travel to not enable interrupts while it's
    waiting for a message;
 3) rework time-travel to not (just) disable interrupts but
    rather block signals at a lower level while it needs them
    disabled for communicating with the controller.

Finally, since now we can actually spend even virtual time
in interrupts-disabled sections, the delay warning when we
deliver a time-travel delayed interrupt is no longer valid,
things can (and should) now get delayed.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
---
 arch/um/include/shared/irq_user.h |  1 +
 arch/um/include/shared/os.h       |  3 +++
 arch/um/kernel/irq.c              | 38 +++++++++++++++++++++-----
 arch/um/kernel/time.c             | 35 +++++++++---------------
 arch/um/os-Linux/signal.c         | 44 ++++++++++++++++++++++++++++---
 5 files changed, 88 insertions(+), 33 deletions(-)

diff --git a/arch/um/include/shared/irq_user.h b/arch/um/include/shared/irq_user.h
index 07239e801a5b..065829f443ae 100644
--- a/arch/um/include/shared/irq_user.h
+++ b/arch/um/include/shared/irq_user.h
@@ -17,6 +17,7 @@ enum um_irq_type {
 
 struct siginfo;
 extern void sigio_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs);
+void sigio_run_timetravel_handlers(void);
 extern void free_irq_by_fd(int fd);
 extern void deactivate_fd(int fd, int irqnum);
 extern int deactivate_all_fds(void);
diff --git a/arch/um/include/shared/os.h b/arch/um/include/shared/os.h
index 13d86f94cf0f..afd5fbc1c867 100644
--- a/arch/um/include/shared/os.h
+++ b/arch/um/include/shared/os.h
@@ -243,6 +243,9 @@ extern int set_signals_trace(int enable);
 extern int os_is_signal_stack(void);
 extern void deliver_alarm(void);
 extern void register_pm_wake_signal(void);
+extern void block_signals_hard(void);
+extern void unblock_signals_hard(void);
+extern void mark_sigio_pending(void);
 
 /* util.c */
 extern void stack_protections(unsigned long address);
diff --git a/arch/um/kernel/irq.c b/arch/um/kernel/irq.c
index 82af5191e73d..76448b85292f 100644
--- a/arch/um/kernel/irq.c
+++ b/arch/um/kernel/irq.c
@@ -123,7 +123,8 @@ static bool irq_do_timetravel_handler(struct irq_entry *entry,
 #endif
 
 static void sigio_reg_handler(int idx, struct irq_entry *entry, enum um_irq_type t,
-			      struct uml_pt_regs *regs)
+			      struct uml_pt_regs *regs,
+			      bool timetravel_handlers_only)
 {
 	struct irq_reg *reg = &entry->reg[t];
 
@@ -136,18 +137,32 @@ static void sigio_reg_handler(int idx, struct irq_entry *entry, enum um_irq_type
 	if (irq_do_timetravel_handler(entry, t))
 		return;
 
-	if (irqs_suspended)
+#ifdef CONFIG_UML_TIME_TRAVEL_SUPPORT
+	/*
+	 * We can only get here with signals disabled if we have time-travel
+	 * support, otherwise we cannot have the hard handlers that may need
+	 * to run even in interrupts-disabled sections and therefore sigio is
+	 * blocked as well when interrupts are disabled.
+	 */
+	if (!get_signals()) {
+		mark_sigio_pending();
+		return;
+	}
+#endif
+
+	if (timetravel_handlers_only)
 		return;
 
 	irq_io_loop(reg, regs);
 }
 
-void sigio_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs)
+static void _sigio_handler(struct uml_pt_regs *regs,
+			   bool timetravel_handlers_only)
 {
 	struct irq_entry *irq_entry;
 	int n, i;
 
-	if (irqs_suspended && !um_irq_timetravel_handler_used())
+	if (timetravel_handlers_only && !um_irq_timetravel_handler_used())
 		return;
 
 	while (1) {
@@ -172,14 +187,20 @@ void sigio_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs)
 			irq_entry = os_epoll_get_data_pointer(i);
 
 			for (t = 0; t < NUM_IRQ_TYPES; t++)
-				sigio_reg_handler(i, irq_entry, t, regs);
+				sigio_reg_handler(i, irq_entry, t, regs,
+						  timetravel_handlers_only);
 		}
 	}
 
-	if (!irqs_suspended)
+	if (!timetravel_handlers_only)
 		free_irqs();
 }
 
+void sigio_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs)
+{
+	_sigio_handler(regs, irqs_suspended);
+}
+
 static struct irq_entry *get_irq_entry_by_fd(int fd)
 {
 	struct irq_entry *walk;
@@ -467,6 +488,11 @@ int um_request_irq_tt(int irq, int fd, enum um_irq_type type,
 			       devname, dev_id, timetravel_handler);
 }
 EXPORT_SYMBOL(um_request_irq_tt);
+
+void sigio_run_timetravel_handlers(void)
+{
+	_sigio_handler(NULL, true);
+}
 #endif
 
 #ifdef CONFIG_PM_SLEEP
diff --git a/arch/um/kernel/time.c b/arch/um/kernel/time.c
index e0cdb9694fb8..fddd1dec27e6 100644
--- a/arch/um/kernel/time.c
+++ b/arch/um/kernel/time.c
@@ -68,23 +68,15 @@ static void time_travel_handle_message(struct um_timetravel_msg *msg,
 	int ret;
 
 	/*
-	 * Poll outside the locked section (if we're not called to only read
-	 * the response) so we can get interrupts for e.g. virtio while we're
-	 * here, but then we need to lock to not get interrupted between the
-	 * read of the message and write of the ACK.
+	 * We can't unlock here, but interrupt signals with a timetravel_handler
+	 * (see um_request_irq_tt) get to the timetravel_handler anyway.
 	 */
 	if (mode != TTMH_READ) {
-		bool disabled = irqs_disabled();
+		BUG_ON(mode == TTMH_IDLE && !irqs_disabled());
 
-		BUG_ON(mode == TTMH_IDLE && !disabled);
-
-		if (disabled)
-			local_irq_enable();
 		while (os_poll(1, &time_travel_ext_fd) != 0) {
 			/* nothing */
 		}
-		if (disabled)
-			local_irq_disable();
 	}
 
 	ret = os_read_file(time_travel_ext_fd, msg, sizeof(*msg));
@@ -123,15 +115,15 @@ static u64 time_travel_ext_req(u32 op, u64 time)
 		.time = time,
 		.seq = mseq,
 	};
-	unsigned long flags;
 
 	/*
-	 * We need to save interrupts here and only restore when we
-	 * got the ACK - otherwise we can get interrupted and send
-	 * another request while we're still waiting for an ACK, but
-	 * the peer doesn't know we got interrupted and will send
-	 * the ACKs in the same order as the message, but we'd need
-	 * to see them in the opposite order ...
+	 * We need to block even the timetravel handlers of SIGIO here and
+	 * only restore their use when we got the ACK - otherwise we may
+	 * (will) get interrupted by that, try to queue the IRQ for future
+	 * processing and thus send another request while we're still waiting
+	 * for an ACK, but the peer doesn't know we got interrupted and will
+	 * send the ACKs in the same order as the message, but we'd need to
+	 * see them in the opposite order ...
 	 *
 	 * This wouldn't matter *too* much, but some ACKs carry the
 	 * current time (for UM_TIMETRAVEL_GET) and getting another
@@ -140,7 +132,7 @@ static u64 time_travel_ext_req(u32 op, u64 time)
 	 * The sequence number assignment that happens here lets us
 	 * debug such message handling issues more easily.
 	 */
-	local_irq_save(flags);
+	block_signals_hard();
 	os_write_file(time_travel_ext_fd, &msg, sizeof(msg));
 
 	while (msg.op != UM_TIMETRAVEL_ACK)
@@ -152,7 +144,7 @@ static u64 time_travel_ext_req(u32 op, u64 time)
 
 	if (op == UM_TIMETRAVEL_GET)
 		time_travel_set_time(msg.time);
-	local_irq_restore(flags);
+	unblock_signals_hard();
 
 	return msg.time;
 }
@@ -352,9 +344,6 @@ void deliver_time_travel_irqs(void)
 	while ((e = list_first_entry_or_null(&time_travel_irqs,
 					     struct time_travel_event,
 					     list))) {
-		WARN(e->time != time_travel_time,
-		     "time moved from %lld to %lld before IRQ delivery\n",
-		     time_travel_time, e->time);
 		list_del(&e->list);
 		e->pending = false;
 		e->fn(e);
diff --git a/arch/um/os-Linux/signal.c b/arch/um/os-Linux/signal.c
index 96f511d1aabe..dc73c6a94d60 100644
--- a/arch/um/os-Linux/signal.c
+++ b/arch/um/os-Linux/signal.c
@@ -63,15 +63,19 @@ static void sig_handler_common(int sig, struct siginfo *si, mcontext_t *mc)
 #define SIGALRM_MASK (1 << SIGALRM_BIT)
 
 static int signals_enabled;
+#ifdef UML_CONFIG_UML_TIME_TRAVEL_SUPPORT
+static int signals_blocked;
+#else
+#define signals_blocked (!signals_enabled)
+#endif
 static unsigned int signals_pending;
 static unsigned int signals_active = 0;
 
 void sig_handler(int sig, struct siginfo *si, mcontext_t *mc)
 {
-	int enabled;
+	int enabled = signals_enabled;
 
-	enabled = signals_enabled;
-	if (!enabled && (sig == SIGIO)) {
+	if (signals_blocked && (sig == SIGIO)) {
 		signals_pending |= SIGIO_MASK;
 		return;
 	}
@@ -99,7 +103,7 @@ void timer_alarm_handler(int sig, struct siginfo *unused_si, mcontext_t *mc)
 	int enabled;
 
 	enabled = signals_enabled;
-	if (!signals_enabled) {
+	if (!signals_enabled || signals_blocked) {
 		signals_pending |= SIGALRM_MASK;
 		return;
 	}
@@ -368,6 +372,38 @@ int set_signals_trace(int enable)
 	return ret;
 }
 
+#ifdef UML_CONFIG_UML_TIME_TRAVEL_SUPPORT
+void mark_sigio_pending(void)
+{
+	signals_pending |= SIGIO_MASK;
+}
+
+void block_signals_hard(void)
+{
+	if (signals_blocked)
+		return;
+	signals_blocked = 1;
+	barrier();
+}
+
+void unblock_signals_hard(void)
+{
+	if (!signals_blocked)
+		return;
+
+	if (signals_pending && signals_enabled) {
+		/* this is a bit inefficient, but that's not really important */
+		block_signals();
+		unblock_signals();
+	} else if (signals_pending & SIGIO_MASK) {
+		/* we need to run time-travel handlers even if not enabled */
+		sigio_run_timetravel_handlers();
+	}
+
+	signals_blocked = 0;
+}
+#endif
+
 int os_is_signal_stack(void)
 {
 	stack_t ss;
-- 
2.26.2