From 9ec23531fd48031d1b6ca5366f5f967d17a8bc28 Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Mon, 11 May 2015 17:52:07 +0200 Subject: [PATCH] sched/preempt, mm/fault: Trigger might_sleep() in might_fault() with disabled pagefaults Commit 662bbcb2747c ("mm, sched: Allow uaccess in atomic with pagefault_disable()") removed might_sleep() checks for all user access code (that uses might_fault()). The reason was to disable wrong "sleep in atomic" warnings in the following scenario: pagefault_disable() rc = copy_to_user(...) pagefault_enable() Which is valid, as pagefault_disable() increments the preempt counter and therefore disables the pagefault handler. copy_to_user() will not sleep and return an error code if a page is not available. However, as all might_sleep() checks are removed, CONFIG_DEBUG_ATOMIC_SLEEP would no longer detect the following scenario: spin_lock(&lock); rc = copy_to_user(...) spin_unlock(&lock) If the kernel is compiled with preemption turned on, preempt_disable() will make in_atomic() detect disabled preemption. The fault handler would correctly never sleep on user access. However, with preemption turned off, preempt_disable() is usually a NOP (with !CONFIG_PREEMPT_COUNT), therefore in_atomic() will not be able to detect disabled preemption nor disabled pagefaults. The fault handler could sleep. We really want to enable CONFIG_DEBUG_ATOMIC_SLEEP checks for user access functions again, otherwise we can end up with horrible deadlocks. Root of all evil is that pagefault_disable() acts almost as preempt_disable(), depending on preemption being turned on/off. As we now have pagefault_disabled(), we can use it to distinguish whether user acces functions might sleep. Convert might_fault() into a makro that calls __might_fault(), to allow proper file + line messages in case of a might_sleep() warning. Reviewed-and-tested-by: Thomas Gleixner Signed-off-by: David Hildenbrand Signed-off-by: Peter Zijlstra (Intel) Cc: David.Laight@ACULAB.COM Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: airlied@linux.ie Cc: akpm@linux-foundation.org Cc: benh@kernel.crashing.org Cc: bigeasy@linutronix.de Cc: borntraeger@de.ibm.com Cc: daniel.vetter@intel.com Cc: heiko.carstens@de.ibm.com Cc: herbert@gondor.apana.org.au Cc: hocko@suse.cz Cc: hughd@google.com Cc: mst@redhat.com Cc: paulus@samba.org Cc: ralf@linux-mips.org Cc: schwidefsky@de.ibm.com Cc: yang.shi@windriver.com Link: http://lkml.kernel.org/r/1431359540-32227-3-git-send-email-dahi@linux.vnet.ibm.com Signed-off-by: Ingo Molnar --- include/linux/kernel.h | 3 ++- mm/memory.c | 18 ++++++------------ 2 files changed, 8 insertions(+), 13 deletions(-) diff --git a/include/linux/kernel.h b/include/linux/kernel.h index 3a5b48e52a9e..060dd7b61c6d 100644 --- a/include/linux/kernel.h +++ b/include/linux/kernel.h @@ -244,7 +244,8 @@ static inline u32 reciprocal_scale(u32 val, u32 ep_ro) #if defined(CONFIG_MMU) && \ (defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP)) -void might_fault(void); +#define might_fault() __might_fault(__FILE__, __LINE__) +void __might_fault(const char *file, int line); #else static inline void might_fault(void) { } #endif diff --git a/mm/memory.c b/mm/memory.c index 22e037e3364e..17734c3c1183 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3737,7 +3737,7 @@ void print_vma_addr(char *prefix, unsigned long ip) } #if defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP) -void might_fault(void) +void __might_fault(const char *file, int line) { /* * Some code (nfs/sunrpc) uses socket ops on kernel memory while @@ -3747,21 +3747,15 @@ void might_fault(void) */ if (segment_eq(get_fs(), KERNEL_DS)) return; - - /* - * it would be nicer only to annotate paths which are not under - * pagefault_disable, however that requires a larger audit and - * providing helpers like get_user_atomic. - */ - if (in_atomic()) + if (pagefault_disabled()) return; - - __might_sleep(__FILE__, __LINE__, 0); - + __might_sleep(file, line, 0); +#if defined(CONFIG_DEBUG_ATOMIC_SLEEP) if (current->mm) might_lock_read(¤t->mm->mmap_sem); +#endif } -EXPORT_SYMBOL(might_fault); +EXPORT_SYMBOL(__might_fault); #endif #if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)