[PATCH 4/7] kcsan: Reduce get_ctx() uses in kcsan_found_watchpoint()

From: Marco Elver
Date: Mon Jun 07 2021 - 08:57:19 EST


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@xxxxxxxxxx>
---
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 d92977ede7e1..906100923b88 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:
--
2.32.0.rc1.229.g3e70b5a671-goog