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Subject[tip: x86/entry] x86/entry: Provide idtentry_entry/exit_cond_rcu()
The following commit has been merged into the x86/entry branch of tip:

Commit-ID: 8fbf48a5cdb83a1ae4285920713facef72639641
Gitweb: https://git.kernel.org/tip/8fbf48a5cdb83a1ae4285920713facef72639641
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Thu, 21 May 2020 22:05:17 +02:00
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Tue, 26 May 2020 19:06:27 +02:00

x86/entry: Provide idtentry_entry/exit_cond_rcu()

After a lengthy discussion [1] it turned out that RCU does not need a full
rcu_irq_enter/exit() when RCU is already watching. All it needs if
NOHZ_FULL is active is to check whether the tick needs to be restarted.

This allows to avoid a separate variant for the pagefault handler which
cannot invoke rcu_irq_enter() on a kernel pagefault which might sleep.

The cond_rcu argument is only temporary and will be removed once the
existing users of idtentry_enter/exit() have been cleaned up. After that
the code can be significantly simplified.

[ mingo: Simplified the control flow ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: "Paul E. McKenney" <paulmck@kernel.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: [1] https://lkml.kernel.org/r/20200515235125.628629605@linutronix.de
Link: https://lore.kernel.org/r/20200521202117.181397835@linutronix.de
---
arch/x86/entry/common.c | 79 +++++++++++++++++++++++++-------
arch/x86/include/asm/idtentry.h | 14 +++++-
2 files changed, 76 insertions(+), 17 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 9ebe334..a7f5846 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -512,8 +512,10 @@ SYSCALL_DEFINE0(ni_syscall)
}

/**
- * idtentry_enter - Handle state tracking on idtentry
+ * idtentry_enter_cond_rcu - Handle state tracking on idtentry with conditional
+ * RCU handling
* @regs: Pointer to pt_regs of interrupted context
+ * @cond_rcu: Invoke rcu_irq_enter() only if RCU is not watching
*
* Invokes:
* - lockdep irqflag state tracking as low level ASM entry disabled
@@ -521,40 +523,84 @@ SYSCALL_DEFINE0(ni_syscall)
*
* - Context tracking if the exception hit user mode.
*
- * - RCU notification if the exception hit kernel mode.
- *
* - The hardirq tracer to keep the state consistent as low level ASM
* entry disabled interrupts.
+ *
+ * For kernel mode entries RCU handling is done conditional. If RCU is
+ * watching then the only RCU requirement is to check whether the tick has
+ * to be restarted. If RCU is not watching then rcu_irq_enter() has to be
+ * invoked on entry and rcu_irq_exit() on exit.
+ *
+ * Avoiding the rcu_irq_enter/exit() calls is an optimization but also
+ * solves the problem of kernel mode pagefaults which can schedule, which
+ * is not possible after invoking rcu_irq_enter() without undoing it.
+ *
+ * For user mode entries enter_from_user_mode() must be invoked to
+ * establish the proper context for NOHZ_FULL. Otherwise scheduling on exit
+ * would not be possible.
+ *
+ * Returns: True if RCU has been adjusted on a kernel entry
+ * False otherwise
+ *
+ * The return value must be fed into the rcu_exit argument of
+ * idtentry_exit_cond_rcu().
*/
-void noinstr idtentry_enter(struct pt_regs *regs)
+bool noinstr idtentry_enter_cond_rcu(struct pt_regs *regs, bool cond_rcu)
{
if (user_mode(regs)) {
enter_from_user_mode();
- } else {
+ return false;
+ }
+
+ if (!cond_rcu || !__rcu_is_watching()) {
+ /*
+ * If RCU is not watching then the same careful
+ * sequence vs. lockdep and tracing is required
+ * as in enter_from_user_mode().
+ *
+ * This only happens for IRQs that hit the idle
+ * loop, i.e. if idle is not using MWAIT.
+ */
lockdep_hardirqs_off(CALLER_ADDR0);
rcu_irq_enter();
instrumentation_begin();
trace_hardirqs_off_prepare();
instrumentation_end();
+
+ return true;
}
+
+ /*
+ * If RCU is watching then RCU only wants to check
+ * whether it needs to restart the tick in NOHZ
+ * mode.
+ */
+ instrumentation_begin();
+ rcu_irq_enter_check_tick();
+ /* Use the combo lockdep/tracing function */
+ trace_hardirqs_off();
+ instrumentation_end();
+
+ return false;
}

/**
- * idtentry_exit - Common code to handle return from exceptions
+ * idtentry_exit_cond_rcu - Handle return from exception with conditional RCU
+ * handling
* @regs: Pointer to pt_regs (exception entry regs)
+ * @rcu_exit: Invoke rcu_irq_exit() if true
*
* Depending on the return target (kernel/user) this runs the necessary
- * preemption and work checks if possible and required and returns to
+ * preemption and work checks if possible and reguired and returns to
* the caller with interrupts disabled and no further work pending.
*
* This is the last action before returning to the low level ASM code which
* just needs to return to the appropriate context.
*
- * Invoked by all exception/interrupt IDTENTRY handlers which are not
- * returning through the paranoid exit path (all except NMI, #DF and the IST
- * variants of #MC and #DB) and are therefore on the thread stack.
+ * Counterpart to idtentry_enter_cond_rcu(). The return value of the entry
+ * function must be fed into the @rcu_exit argument.
*/
-void noinstr idtentry_exit(struct pt_regs *regs)
+void noinstr idtentry_exit_cond_rcu(struct pt_regs *regs, bool rcu_exit)
{
lockdep_assert_irqs_disabled();

@@ -580,7 +626,8 @@ void noinstr idtentry_exit(struct pt_regs *regs)
if (IS_ENABLED(CONFIG_DEBUG_ENTRY))
WARN_ON_ONCE(!on_thread_stack());
instrumentation_begin();
- rcu_irq_exit_preempt();
+ if (rcu_exit)
+ rcu_irq_exit_preempt();
if (need_resched())
preempt_schedule_irq();
/* Covers both tracing and lockdep */
@@ -602,10 +649,12 @@ void noinstr idtentry_exit(struct pt_regs *regs)
trace_hardirqs_on_prepare();
lockdep_hardirqs_on_prepare(CALLER_ADDR0);
instrumentation_end();
- rcu_irq_exit();
+ if (rcu_exit)
+ rcu_irq_exit();
lockdep_hardirqs_on(CALLER_ADDR0);
} else {
- /* IRQ flags state is correct already. Just tell RCU */
- rcu_irq_exit();
+ /* IRQ flags state is correct already. Just tell RCU. */
+ if (rcu_exit)
+ rcu_irq_exit();
}
}
diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index ce97478..a116b80 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -7,8 +7,18 @@

#ifndef __ASSEMBLY__

-void idtentry_enter(struct pt_regs *regs);
-void idtentry_exit(struct pt_regs *regs);
+bool idtentry_enter_cond_rcu(struct pt_regs *regs, bool cond_rcu);
+void idtentry_exit_cond_rcu(struct pt_regs *regs, bool rcu_exit);
+
+static __always_inline void idtentry_enter(struct pt_regs *regs)
+{
+ idtentry_enter_cond_rcu(regs, false);
+}
+
+static __always_inline void idtentry_exit(struct pt_regs *regs)
+{
+ idtentry_exit_cond_rcu(regs, true);
+}

/**
* DECLARE_IDTENTRY - Declare functions for simple IDT entry points
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