@@ -93,6 +93,10 @@ struct ww_acquire_ctx {
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif
+#ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
+ unsigned deadlock_inject_interval;
+ unsigned deadlock_inject_countdown;
+#endif
};
struct ww_mutex {
@@ -278,6 +282,10 @@ static inline void ww_acquire_init(struct ww_acquire_ctx *ctx,
&ww_class->acquire_key, 0);
mutex_acquire(&ctx->dep_map, 0, 0, _RET_IP_);
#endif
+#ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
+ ctx->deadlock_inject_interval = ctx->stamp & 0xf;
+ ctx->deadlock_inject_countdown = ctx->deadlock_inject_interval;
+#endif
}
/**
@@ -827,6 +827,35 @@ int __sched mutex_trylock(struct mutex *lock)
EXPORT_SYMBOL(mutex_trylock);
#ifndef CONFIG_DEBUG_LOCK_ALLOC
+
+#ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
+static int __sched
+ww_mutex_deadlock_injection(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
+{
+ if (ctx->deadlock_inject_countdown-- == 0) {
+ tmp = ctx->deadlock_inject_interval;
+ if (tmp > UINT_MAX/4)
+ tmp = UINT_MAX;
+ else
+ tmp = tmp*2 + tmp + tmp/2;
+
+ ctx->deadlock_inject_interval = tmp;
+ ctx->deadlock_inject_countdown = tmp;
+
+ ww_mutex_unlock(lock);
+
+ return -EDEADLK;
+ }
+
+ return 0;
+}
+#else
+static int __sched
+ww_mutex_deadlock_injection(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
+{
+ return 0;
+}
+#endif
int __sched
ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
{
@@ -839,6 +868,7 @@ ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
if (likely(!ret)) {
ww_mutex_set_context_fastpath(lock, ctx);
mutex_set_owner(&lock->base);
+ return ww_mutex_deadlock_injection(lock, ctx);
} else
ret = __ww_mutex_lock_slowpath(lock, ctx);
return ret;
@@ -857,6 +887,7 @@ ww_mutex_lock_interruptible(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
if (likely(!ret)) {
ww_mutex_set_context_fastpath(lock, ctx);
mutex_set_owner(&lock->base);
+ return ww_mutex_deadlock_injection(lock, ctx);
} else
ret = __ww_mutex_lock_interruptible_slowpath(lock, ctx);
return ret;
@@ -547,6 +547,16 @@ config DEBUG_MUTEXES
This feature allows mutex semantics violations to be detected and
reported.
+config DEBUG_WW_MUTEX_SLOWPATH
+ bool "Wait/wound mutex debugging: Slowpath testing"
+ depends on DEBUG_KERNEL
+ help
+ This feature enables slowpath testing for w/w mutex users by
+ injecting additional -EDEADLK wound/backoff cases. Together with
+ the full mutex checks enabled with (CONFIG_PROVE_LOCKING) this
+ will test all possible w/w mutex interface abuse with the
+ exception of simply not acquiring all the required locks.
+
config DEBUG_LOCK_ALLOC
bool "Lock debugging: detect incorrect freeing of live locks"
depends on DEBUG_KERNEL && TRACE_IRQFLAGS_SUPPORT && STACKTRACE_SUPPORT && LOCKDEP_SUPPORT
Injects EDEADLK conditions at pseudo-random interval, with exponential backoff up to UINT_MAX (to ensure that every lock operation still completes in a reasonable time). This way we can test the wound slowpath even for ww mutex users where contention is never expected, and the ww deadlock avoidance algorithm is only needed for correctness against malicious userspace. An example would be protecting kernel modesetting properties, which thanks to single-threaded X isn't really expected to contend, ever. I've looked into using the CONFIG_FAULT_INJECTION infrastructure, but decided against it for two reasons: - EDEADLK handling is mandatory for ww mutex users and should never affect the outcome of a syscall. This is in contrast to -ENOMEM injection. So fine configurability isn't required. - The fault injection framework only allows to set a simple probability for failure. Now the probability that a ww mutex acquire stage with N locks will never complete (due to too many injected EDEADLK backoffs) is zero. But the expected number of ww_mutex_lock operations for the completely uncontended case would be O(exp(N)). The per-acuiqire ctx exponential backoff solution choosen here only results in O(log N) overhead due to injection and so O(log N * N) lock operations. This way we can fail with high probability (and so have good test coverage even for fancy backoff and lock acquisition paths) without running into patalogical cases. Note that EDEADLK will only ever be injected when we managed to acquire the lock. This prevents any behaviour changes for users which rely on the EALREADY semantics. Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> --- include/linux/mutex.h | 8 ++++++++ kernel/mutex.c | 31 +++++++++++++++++++++++++++++++ lib/Kconfig.debug | 10 ++++++++++ 3 files changed, 49 insertions(+)