From: Frederic Weisbecker <frederic@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
Frederic Weisbecker <frederic@kernel.org>,
"Paul E . McKenney" <paulmck@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Phil Auld <pauld@redhat.com>,
Marcelo Tosatti <mtosatti@redhat.com>
Subject: [PATCH 2/5] context_tracking: Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK
Date: Tue, 17 Nov 2020 16:16:34 +0100 [thread overview]
Message-ID: <20201117151637.259084-3-frederic@kernel.org> (raw)
In-Reply-To: <20201117151637.259084-1-frederic@kernel.org>
The typical steps with context tracking are:
1) Task runs in userspace
2) Task enters the kernel (syscall/exception/IRQ)
3) Task switches from context tracking state CONTEXT_USER to
CONTEXT_KERNEL (user_exit())
4) Task does stuff in kernel
5) Task switches from context tracking state CONTEXT_KERNEL to
CONTEXT_USER (user_enter())
6) Task exits the kernel
If an exception fires between 5) and 6), the pt_regs and the context
tracking disagree on the context of the faulted/trapped instruction.
CONTEXT_KERNEL must be set before the exception handler, that's
unconditional for those handlers that want to be able to call into
schedule(), but CONTEXT_USER must be restored when the exception exits
whereas pt_regs tells that we are resuming to kernel space.
This can't be fixed with storing the context tracking state in a per-cpu
or per-task variable since another exception may fire onto the current
one and overwrite the saved state. Also the task can schedule. So it
has to be stored in a per task stack.
This is how exception_enter()/exception_exit() paper over the problem:
5) Task switches from context tracking state CONTEXT_KERNEL to
CONTEXT_USER (user_enter())
5.1) Exception fires
5.2) prev_state = exception_enter() // save CONTEXT_USER to prev_state
// and set CONTEXT_KERNEL
5.3) Exception handler
5.4) exception_enter(prev_state) // restore CONTEXT_USER
5.5) Exception resumes
6) Task exits the kernel
The condition to live without exception_enter()/exception_exit() is to
forbid exceptions and IRQs between 2) and 3) and between 5) and 6), or if
any is allowed to trigger, it won't call into context tracking, eg: NMIs,
and it won't schedule. These requirements are met by architectures
supporting CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK and those can
therefore afford not to implement this hack.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Phil Auld <pauld@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
include/linux/context_tracking.h | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index d53cd331c4dd..bceb06498521 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -51,7 +51,8 @@ static inline enum ctx_state exception_enter(void)
{
enum ctx_state prev_ctx;
- if (!context_tracking_enabled())
+ if (IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) ||
+ !context_tracking_enabled())
return 0;
prev_ctx = this_cpu_read(context_tracking.state);
@@ -63,7 +64,8 @@ static inline enum ctx_state exception_enter(void)
static inline void exception_exit(enum ctx_state prev_ctx)
{
- if (context_tracking_enabled()) {
+ if (!IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) &&
+ context_tracking_enabled()) {
if (prev_ctx != CONTEXT_KERNEL)
context_tracking_enter(prev_ctx);
}
--
2.25.1
next prev parent reply other threads:[~2020-11-17 15:17 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-17 15:16 [PATCH 0/5] context_tracking: Flatter archs not using exception_enter/exit() v3 Frederic Weisbecker
2020-11-17 15:16 ` [PATCH 1/5] context_tracking: Introduce HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
2020-11-20 12:44 ` [tip: core/entry] " tip-bot2 for Frederic Weisbecker
2020-11-17 15:16 ` Frederic Weisbecker [this message]
2020-11-20 12:44 ` [tip: core/entry] context_tracking: Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK tip-bot2 for Frederic Weisbecker
2020-11-17 15:16 ` [PATCH 3/5] sched: Detect call to schedule from critical entry code Frederic Weisbecker
2020-11-20 12:44 ` [tip: core/entry] " tip-bot2 for Frederic Weisbecker
2020-11-17 15:16 ` [PATCH 4/5] context_tracking: Only define schedule_user() on !HAVE_CONTEXT_TRACKING_OFFSTACK archs Frederic Weisbecker
2020-11-20 12:44 ` [tip: core/entry] " tip-bot2 for Frederic Weisbecker
2020-11-17 15:16 ` [PATCH 5/5] x86: Support HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
2020-11-20 12:44 ` [tip: core/entry] " tip-bot2 for Frederic Weisbecker
2020-11-18 7:39 ` [PATCH 0/5] context_tracking: Flatter archs not using exception_enter/exit() v3 Peter Zijlstra
2020-11-18 13:48 ` Frederic Weisbecker
2020-11-18 14:05 ` Peter Zijlstra
2020-11-18 14:20 ` Frederic Weisbecker
-- strict thread matches above, loose matches on Subject: below --
2020-10-27 15:08 [PATCH 0/5] context_tracking: Flatter archs not using exception_enter/exit() v2 Frederic Weisbecker
2020-10-27 15:08 ` [PATCH 2/5] context_tracking: Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
2020-11-11 14:32 ` Peter Zijlstra
2020-11-11 22:20 ` Frederic Weisbecker
2020-10-05 10:49 [PATCH 0/5] context_tracking: Flatter archs not using exception_enter/exit() Frederic Weisbecker
2020-10-05 10:49 ` [PATCH 2/5] context_tracking: Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
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