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kcsan: Reduce get_ctx() uses in kcsan_found_watchpoint()

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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>
This commit is contained in:
Marco Elver 2021-06-07 14:56:50 +02:00 committed by Paul E. McKenney
parent e675d2533a
commit 08cac60494
1 changed files with 16 additions and 10 deletions
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26
kernel/kcsan/core.c
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@ -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: