Commit 08cac604 authored by Marco Elver's avatar Marco Elver Committed by Paul E. McKenney
Browse files

kcsan: Reduce get_ctx() uses in kcsan_found_watchpoint() 



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: default avatarMarco Elver <elver@google.com>
Acked-by: default avatarMark Rutland <mark.rutland@arm.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent e675d253

kernel/kcsan/core.c

+16 −10
Original line number Diff line number Diff line
@@ -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: