On Wed, Aug 19, 2015 at 06:19:43AM +0930, Rusty Russell wrote: > Indeed! That comment is wrong, and your fix is good. > > Care to S-O-B on it?
Of course, here goes. --- Subject: module: Fix locking in symbol_put_addr() Laura 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] 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: Laura Abbott <labb...@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> --- kernel/module.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/kernel/module.c b/kernel/module.c index b86b7bf1be38..8f051a106676 100644 --- a/kernel/module.c +++ b/kernel/module.c @@ -1063,11 +1063,15 @@ void symbol_put_addr(void *addr) if (core_kernel_text(a)) return; - /* module_text_address is safe here: we're supposed to have reference - * to module from symbol_get, so it can't go away. */ + /* + * Even though we hold a reference on the module; we still need to + * disable preemption in order to safely traverse the data structure. + */ + preempt_disable(); modaddr = __module_text_address(a); BUG_ON(!modaddr); module_put(modaddr); + preempt_enable(); } EXPORT_SYMBOL_GPL(symbol_put_addr); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/