Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933657Ab3D3UgA (ORCPT ); Tue, 30 Apr 2013 16:36:00 -0400 Received: from mail-wi0-f172.google.com ([209.85.212.172]:48765 "EHLO mail-wi0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933034Ab3D3Uf6 (ORCPT ); Tue, 30 Apr 2013 16:35:58 -0400 From: Daniel Vetter To: LKML Cc: linux-arch@vger.kernel.org, peterz@infradead.org, x86@kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, robclark@gmail.com, rostedt@goodmis.org, daniel@ffwll.ch, tglx@linutronix.de, mingo@elte.hu, linux-media@vger.kernel.org, Daniel Vetter Subject: [PATCH] [RFC] mutex: w/w mutex slowpath debugging Date: Tue, 30 Apr 2013 22:38:56 +0200 Message-Id: <1367354336-13719-1-git-send-email-daniel.vetter@ffwll.ch> X-Mailer: git-send-email 1.7.10.4 In-Reply-To: <1367350179.30667.70.camel@gandalf.local.home> References: <1367350179.30667.70.camel@gandalf.local.home> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5529 Lines: 156 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. v2: Drop the cargo-culted __sched (I should read docs next time around) and annotate the non-debug case with inline to prevent gcc from doing something horrible. Cc: Steven Rostedt Signed-off-by: Daniel Vetter --- include/linux/mutex.h | 8 ++++++++ kernel/mutex.c | 32 ++++++++++++++++++++++++++++++++ lib/Kconfig.debug | 10 ++++++++++ 3 files changed, 50 insertions(+) diff --git a/include/linux/mutex.h b/include/linux/mutex.h index 004f863..82d56ec 100644 --- a/include/linux/mutex.h +++ b/include/linux/mutex.h @@ -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 } /** diff --git a/kernel/mutex.c b/kernel/mutex.c index 66807c7..816fdfc 100644 --- a/kernel/mutex.c +++ b/kernel/mutex.c @@ -827,6 +827,36 @@ int __sched mutex_trylock(struct mutex *lock) EXPORT_SYMBOL(mutex_trylock); #ifndef CONFIG_DEBUG_LOCK_ALLOC + +#ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH +static int +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 inline int +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 +869,7 @@ 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 +888,7 @@ 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; diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index 28be08c..8c41f73 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -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 -- 1.7.10.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/