From 0a91dc191b6a008f340dae5d4c07a6c50ce41ece Mon Sep 17 00:00:00 2001 From: Daniel Vetter Date: Thu, 20 Jun 2013 13:31:17 +0200 Subject: mutex: Add w/w mutex slowpath debugging 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 Signed-off-by: Maarten Lankhorst Acked-by: Peter Zijlstra Cc: dri-devel@lists.freedesktop.org Cc: linaro-mm-sig@lists.linaro.org Cc: rostedt@goodmis.org Cc: daniel@ffwll.ch Cc: Linus Torvalds Cc: Andrew Morton Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/20130620113117.4001.21681.stgit@patser Signed-off-by: Ingo Molnar --- include/linux/mutex.h | 8 ++++++++ 1 file changed, 8 insertions(+) (limited to 'include') diff --git a/include/linux/mutex.h b/include/linux/mutex.h index a56b0ccc8a6..3793ed7feee 100644 --- a/include/linux/mutex.h +++ b/include/linux/mutex.h @@ -98,6 +98,10 @@ 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 { @@ -283,6 +287,10 @@ 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 = 1; + ctx->deadlock_inject_countdown = ctx->stamp & 0xf; +#endif } /** -- cgit v1.2.3