linux-kernel - [tip: locking/core] kcsan: Make KCSAN compatible with new IRQ state tracking

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Date:   Sat, 11 Jul 2020 10:09:57 -0000
From:   "tip-bot2 for Marco Elver" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>, Ingo Molnar <mingo@...nel.org>,
        x86 <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: [tip: locking/core] kcsan: Make KCSAN compatible with new IRQ state tracking

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     248591f5d257a19c1cba9ab9da3536bfbc2f0149
Gitweb:        https://git.kernel.org/tip/248591f5d257a19c1cba9ab9da3536bfbc2f0149
Author:        Marco Elver <elver@...gle.com>
AuthorDate:    Wed, 24 Jun 2020 13:32:46 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Fri, 10 Jul 2020 12:00:00 +02:00

kcsan: Make KCSAN compatible with new IRQ state tracking

The new IRQ state tracking code does not honor lockdep_off(), and as
such we should again permit tracing by using non-raw functions in
core.c. Update the lockdep_off() comment in report.c, to reflect the
fact there is still a potential risk of deadlock due to using printk()
from scheduler code.

Suggested-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Marco Elver <elver@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Ingo Molnar <mingo@...nel.org>
Link: https://lkml.kernel.org/r/20200624113246.GA170324@elver.google.com
---
 kernel/kcsan/core.c   |  5 ++---
 kernel/kcsan/report.c |  9 +++++----
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 15f6794..732623c 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -397,8 +397,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	}
 
 	if (!kcsan_interrupt_watcher)
-		/* Use raw to avoid lockdep recursion via IRQ flags tracing. */
-		raw_local_irq_save(irq_flags);
+		local_irq_save(irq_flags);
 
 	watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
 	if (watchpoint == NULL) {
@@ -539,7 +538,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	kcsan_counter_dec(KCSAN_COUNTER_USED_WATCHPOINTS);
 out_unlock:
 	if (!kcsan_interrupt_watcher)
-		raw_local_irq_restore(irq_flags);
+		local_irq_restore(irq_flags);
 out:
 	user_access_restore(ua_flags);
 }
diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index ac5f834..6b2fb1a 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -606,10 +606,11 @@ void kcsan_report(const volatile void *ptr, size_t size, int access_type,
 		goto out;
 
 	/*
-	 * With TRACE_IRQFLAGS, lockdep's IRQ trace state becomes corrupted if
-	 * we do not turn off lockdep here; this could happen due to recursion
-	 * into lockdep via KCSAN if we detect a race in utilities used by
-	 * lockdep.
+	 * Because we may generate reports when we're in scheduler code, the use
+	 * of printk() could deadlock. Until such time that all printing code
+	 * called in print_report() is scheduler-safe, accept the risk, and just
+	 * get our message out. As such, also disable lockdep to hide the
+	 * warning, and avoid disabling lockdep for the rest of the kernel.
 	 */
 	lockdep_off();