Commit 44656d3d authored by Marco Elver's avatar Marco Elver Committed by Paul E. McKenney

kcsan: Add current->state to implicitly atomic accesses

Add volatile current->state to list of implicitly atomic accesses. This
is in preparation to eventually enable KCSAN on kernel/sched (which
currently still has KCSAN_SANITIZE := n).

Since accesses that match the special check in atomic.h are rare, it
makes more sense to move this check to the slow-path, avoiding the
additional compare in the fast-path. With the microbenchmark, a speedup
of ~6% is measured.
Signed-off-by: default avatarMarco Elver <elver@google.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 2402d0ea
......@@ -4,24 +4,17 @@
#define _KERNEL_KCSAN_ATOMIC_H
#include <linux/jiffies.h>
#include <linux/sched.h>
/*
* Helper that returns true if access to @ptr should be considered an atomic
* access, even though it is not explicitly atomic.
*
* List all volatile globals that have been observed in races, to suppress
* data race reports between accesses to these variables.
*
* For now, we assume that volatile accesses of globals are as strong as atomic
* accesses (READ_ONCE, WRITE_ONCE cast to volatile). The situation is still not
* entirely clear, as on some architectures (Alpha) READ_ONCE/WRITE_ONCE do more
* than cast to volatile. Eventually, we hope to be able to remove this
* function.
* Special rules for certain memory where concurrent conflicting accesses are
* common, however, the current convention is to not mark them; returns true if
* access to @ptr should be considered atomic. Called from slow-path.
*/
static __always_inline bool kcsan_is_atomic(const volatile void *ptr)
static bool kcsan_is_atomic_special(const volatile void *ptr)
{
/* only jiffies for now */
return ptr == &jiffies;
/* volatile globals that have been observed in data races. */
return ptr == &jiffies || ptr == &current->state;
}
#endif /* _KERNEL_KCSAN_ATOMIC_H */
......@@ -188,12 +188,13 @@ static __always_inline struct kcsan_ctx *get_ctx(void)
return in_task() ? &current->kcsan_ctx : raw_cpu_ptr(&kcsan_cpu_ctx);
}
/* Rules for generic atomic accesses. Called from fast-path. */
static __always_inline bool
is_atomic(const volatile void *ptr, size_t size, int type)
{
struct kcsan_ctx *ctx;
if ((type & KCSAN_ACCESS_ATOMIC) != 0)
if (type & KCSAN_ACCESS_ATOMIC)
return true;
/*
......@@ -201,16 +202,16 @@ is_atomic(const volatile void *ptr, size_t size, int type)
* as atomic. This allows using them also in atomic regions, such as
* seqlocks, without implicitly changing their semantics.
*/
if ((type & KCSAN_ACCESS_ASSERT) != 0)
if (type & KCSAN_ACCESS_ASSERT)
return false;
if (IS_ENABLED(CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC) &&
(type & KCSAN_ACCESS_WRITE) != 0 && size <= sizeof(long) &&
(type & KCSAN_ACCESS_WRITE) && size <= sizeof(long) &&
IS_ALIGNED((unsigned long)ptr, size))
return true; /* Assume aligned writes up to word size are atomic. */
ctx = get_ctx();
if (unlikely(ctx->atomic_next > 0)) {
if (ctx->atomic_next > 0) {
/*
* Because we do not have separate contexts for nested
* interrupts, in case atomic_next is set, we simply assume that
......@@ -224,10 +225,8 @@ is_atomic(const volatile void *ptr, size_t size, int type)
--ctx->atomic_next; /* in task, or outer interrupt */
return true;
}
if (unlikely(ctx->atomic_nest_count > 0 || ctx->in_flat_atomic))
return true;
return kcsan_is_atomic(ptr);
return ctx->atomic_nest_count > 0 || ctx->in_flat_atomic;
}
static __always_inline bool
......@@ -367,6 +366,15 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
if (!kcsan_is_enabled())
goto out;
/*
* Special atomic rules: unlikely to be true, so we check them here in
* the slow-path, and not in the fast-path in is_atomic(). Call after
* kcsan_is_enabled(), as we may access memory that is not yet
* initialized during early boot.
*/
if (!is_assert && kcsan_is_atomic_special(ptr))
goto out;
if (!check_encodable((unsigned long)ptr, size)) {
kcsan_counter_inc(KCSAN_COUNTER_UNENCODABLE_ACCESSES);
goto out;
......
......@@ -74,25 +74,34 @@ void kcsan_counter_dec(enum kcsan_counter_id id)
*/
static noinline void microbenchmark(unsigned long iters)
{
const struct kcsan_ctx ctx_save = current->kcsan_ctx;
const bool was_enabled = READ_ONCE(kcsan_enabled);
cycles_t cycles;
/* We may have been called from an atomic region; reset context. */
memset(&current->kcsan_ctx, 0, sizeof(current->kcsan_ctx));
/*
* Disable to benchmark fast-path for all accesses, and (expected
* negligible) call into slow-path, but never set up watchpoints.
*/
WRITE_ONCE(kcsan_enabled, false);
pr_info("KCSAN: %s begin | iters: %lu\n", __func__, iters);
cycles = get_cycles();
while (iters--) {
/*
* We can run this benchmark from multiple tasks; this address
* calculation increases likelyhood of some accesses
* overlapping. Make the access type an atomic read, to never
* set up watchpoints and test the fast-path only.
*/
unsigned long addr =
iters % (CONFIG_KCSAN_NUM_WATCHPOINTS * PAGE_SIZE);
__kcsan_check_access((void *)addr, sizeof(long), KCSAN_ACCESS_ATOMIC);
unsigned long addr = iters & ((PAGE_SIZE << 8) - 1);
int type = !(iters & 0x7f) ? KCSAN_ACCESS_ATOMIC :
(!(iters & 0xf) ? KCSAN_ACCESS_WRITE : 0);
__kcsan_check_access((void *)addr, sizeof(long), type);
}
cycles = get_cycles() - cycles;
pr_info("KCSAN: %s end | cycles: %llu\n", __func__, cycles);
WRITE_ONCE(kcsan_enabled, was_enabled);
/* restore context */
current->kcsan_ctx = ctx_save;
}
/*
......
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