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: Marco Elver <elver@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index d92977e..9061009 100644 (file)
--- 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: