On Thu, Nov 27, 2014 at 06:10:17PM +0100, David Hildenbrand wrote:
> Commit 662bbcb2747c2422cf98d3d97619509379eee466 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 invalid return 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, the preempt counter would
> be incremented and copy_to_user() would never sleep. However, with preemption
> turned off, the preempt counter will not be touched, we will therefore sleep in
> atomic context. We really want to enable CONFIG_DEBUG_ATOMIC_SLEEP checks for
> user access functions again, otherwise horrible deadlocks might be hard to debug.
>
> Root of all evil is that pagefault_disable() acted almost as preempt_disable(),
> depending on preemption being turned on/off.
>
> As we now have a fixed pagefault_disable() implementation in place, that uses
> own bits in the preempt counter, we can reenable might_sleep() checks.
>
> This patch reverts commit 662bbcb2747c2422cf98d3d97619509379eee466 taking care
> of the !MMU optimization and the new pagefault_disabled() check.
>
> Signed-off-by: David Hildenbrand <dahi@xxxxxxxxxxxxxxxxxx>
> ---
> include/linux/kernel.h | 9 +++++++--
> mm/memory.c | 15 ++++-----------
> 2 files changed, 11 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 3d770f55..64b5f93 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -225,9 +225,14 @@ static inline u32 reciprocal_scale(u32 val, u32 ep_ro)
> return (u32)(((u64) val * ep_ro) >> 32);
> }
>
> -#if defined(CONFIG_MMU) && \
> - (defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP))
> +#if defined(CONFIG_MMU) && defined(CONFIG_PROVE_LOCKING)
> void might_fault(void);
> +#elif defined(CONFIG_MMU) && defined(CONFIG_DEBUG_ATOMIC_SLEEP)
> +static inline void might_fault(void)
> +{
> + if (unlikely(!pagefault_disabled()))
> + __might_sleep(__FILE__, __LINE__, 0);
This __FILE__/__FILE__ will always point at kernel.h
You want a macro to wrap this up.
> +}
> #else
> static inline void might_fault(void) { }
> #endif
> diff --git a/mm/memory.c b/mm/memory.c
> index 3e50383..0e59db9 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3699,7 +3699,7 @@ void print_vma_addr(char *prefix, unsigned long ip)
> up_read(&mm->mmap_sem);
> }
>
> -#if defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP)
> +#ifdef CONFIG_PROVE_LOCKING
> void might_fault(void)
> {
> /*
> @@ -3711,17 +3711,10 @@ 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())
> - return;
> -
> - __might_sleep(__FILE__, __LINE__, 0);
> + if (unlikely(!pagefault_disabled()))
> + __might_sleep(__FILE__, __LINE__, 0);
>
> - if (current->mm)
> + if (!in_atomic() && current->mm)
> might_lock_read(¤t->mm->mmap_sem);
> }
> EXPORT_SYMBOL(might_fault);
> --
> 1.8.5.5
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