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From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com,
        kernel-team@fb.com, mingo@kernel.org
Cc:     elver@google.com, andreyknvl@google.com, glider@google.com,
        dvyukov@google.com, cai@lca.pw, boqun.feng@gmail.com,
        Mark Rutland <mark.rutland@arm.com>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: [PATCH kcsan 4/8] kcsan: Reduce get_ctx() uses in kcsan_found_watchpoint()
Date:   Wed, 21 Jul 2021 14:08:08 -0700
Message-Id: <20210721210812.844740-4-paulmck@kernel.org>
In-Reply-To: <20210721210726.GA828672@paulmck-ThinkPad-P17-Gen-1>
References: <20210721210726.GA828672@paulmck-ThinkPad-P17-Gen-1>
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From: Marco Elver <elver@google.com>

There are a number get_ctx() calls that are close to each other, which
results in poor codegen (repeated preempt_count loads).

Specifically in kcsan_found_watchpoint() (even though it's a slow-path)
it is beneficial to keep the race-window small until the watchpoint has
actually been consumed to avoid missed opportunities to report a race.

Let's clean it up a bit before we add more code in
kcsan_found_watchpoint().

Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/kcsan/core.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index d92977ede7e17..906100923b888 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -301,9 +301,9 @@ static inline void reset_kcsan_skip(void)
 	this_cpu_write(kcsan_skip, skip_count);
 }
 
-static __always_inline bool kcsan_is_enabled(void)
+static __always_inline bool kcsan_is_enabled(struct kcsan_ctx *ctx)
 {
-	return READ_ONCE(kcsan_enabled) && get_ctx()->disable_count == 0;
+	return READ_ONCE(kcsan_enabled) && !ctx->disable_count;
 }
 
 /* Introduce delay depending on context and configuration. */
@@ -353,10 +353,17 @@ static noinline void kcsan_found_watchpoint(const volatile void *ptr,
 					    atomic_long_t *watchpoint,
 					    long encoded_watchpoint)
 {
+	struct kcsan_ctx *ctx = get_ctx();
 	unsigned long flags;
 	bool consumed;
 
-	if (!kcsan_is_enabled())
+	/*
+	 * We know a watchpoint exists. Let's try to keep the race-window
+	 * between here and finally consuming the watchpoint below as small as
+	 * possible -- avoid unneccessarily complex code until consumed.
+	 */
+
+	if (!kcsan_is_enabled(ctx))
 		return;
 
 	/*
@@ -364,14 +371,12 @@ static noinline void kcsan_found_watchpoint(const volatile void *ptr,
 	 * reporting a race where e.g. the writer set up the watchpoint, but the
 	 * reader has access_mask!=0, we have to ignore the found watchpoint.
 	 */
-	if (get_ctx()->access_mask != 0)
+	if (ctx->access_mask)
 		return;
 
 	/*
-	 * Consume the watchpoint as soon as possible, to minimize the chances
-	 * of !consumed. Consuming the watchpoint must always be guarded by
-	 * kcsan_is_enabled() check, as otherwise we might erroneously
-	 * triggering reports when disabled.
+	 * Consuming the watchpoint must be guarded by kcsan_is_enabled() to
+	 * avoid erroneously triggering reports if the context is disabled.
 	 */
 	consumed = try_consume_watchpoint(watchpoint, encoded_watchpoint);
 
@@ -409,6 +414,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	unsigned long access_mask;
 	enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
 	unsigned long ua_flags = user_access_save();
+	struct kcsan_ctx *ctx = get_ctx();
 	unsigned long irq_flags = 0;
 
 	/*
@@ -417,7 +423,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	 */
 	reset_kcsan_skip();
 
-	if (!kcsan_is_enabled())
+	if (!kcsan_is_enabled(ctx))
 		goto out;
 
 	/*
@@ -489,7 +495,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	 * Re-read value, and check if it is as expected; if not, we infer a
 	 * racy access.
 	 */
-	access_mask = get_ctx()->access_mask;
+	access_mask = ctx->access_mask;
 	new = 0;
 	switch (size) {
 	case 1:
-- 
2.31.1.189.g2e36527f23