• Peter Zijlstra's avatar
    module: Fix locking in symbol_put_addr() · 275d7d44
    Peter Zijlstra authored
    Poma (on the way to another bug) reported an assertion triggering:
      [<ffffffff81150529>] module_assert_mutex_or_preempt+0x49/0x90
      [<ffffffff81150822>] __module_address+0x32/0x150
      [<ffffffff81150956>] __module_text_address+0x16/0x70
      [<ffffffff81150f19>] symbol_put_addr+0x29/0x40
      [<ffffffffa04b77ad>] dvb_frontend_detach+0x7d/0x90 [dvb_core]
    Laura Abbott <labbott@redhat.com> produced a patch which lead us to
    inspect symbol_put_addr(). This function has a comment claiming it
    doesn't need to disable preemption around the module lookup
    because it holds a reference to the module it wants to find, which
    therefore cannot go away.
    This is wrong (and a false optimization too, preempt_disable() is really
    rather cheap, and I doubt any of this is on uber critical paths,
    otherwise it would've retained a pointer to the actual module anyway and
    avoided the second lookup).
    While its true that the module cannot go away while we hold a reference
    on it, the data structure we do the lookup in very much _CAN_ change
    while we do the lookup. Therefore fix the comment and add the
    required preempt_disable().
    Reported-by: default avatarpoma <pomidorabelisima@gmail.com>
    Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: default avatarRusty Russell <rusty@rustcorp.com.au>
    Fixes: a6e6abd5 ("module: remove module_text_address()")
    Cc: stable@kernel.org
module.c 102 KB