• David Hildenbrand's avatar
    sched/preempt, mm/fault: Trigger might_sleep() in might_fault() with disabled pagefaults · 9ec23531
    David Hildenbrand authored
    Commit 662bbcb2 ("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: default avatarThomas Gleixner <tglx@linutronix.de>
    Signed-off-by: default avatarDavid Hildenbrand <dahi@linux.vnet.ibm.com>
    Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Cc: David.Laight@ACULAB.COM
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    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.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    9ec23531
memory.c 104 KB