From: Ingo Molnar Date: Fri, 19 Oct 2007 10:19:26 +0000 (+0200) Subject: x86: fix global_flush_tlb() bug X-Git-Url: http://git.lede-project.org./?a=commitdiff_plain;h=9a24d04a3c26c223f22493492c5c9085b8773d4a;p=openwrt%2Fstaging%2Fblogic.git x86: fix global_flush_tlb() bug While we were reviewing pageattr_32/64.c for unification, Thomas Gleixner noticed the following serious SMP bug in global_flush_tlb(): down_read(&init_mm.mmap_sem); list_replace_init(&deferred_pages, &l); up_read(&init_mm.mmap_sem); this is SMP-unsafe because list_replace_init() done on two CPUs in parallel can corrupt the list. This bug has been introduced about a year ago in the 64-bit tree: commit ea7322decb974a4a3e804f96a0201e893ff88ce3 Author: Andi Kleen Date: Thu Dec 7 02:14:05 2006 +0100 [PATCH] x86-64: Speed and clean up cache flushing in change_page_attr down_read(&init_mm.mmap_sem); - dpage = xchg(&deferred_pages, NULL); + list_replace_init(&deferred_pages, &l); up_read(&init_mm.mmap_sem); the xchg() based version was SMP-safe, but list_replace_init() is not. So this "cleanup" introduced a nasty bug. why this bug never become prominent is a mystery - it can probably be explained with the (still) relative obscurity of the x86_64 architecture. the safe fix for now is to write-lock init_mm.mmap_sem. Signed-off-by: Ingo Molnar Signed-off-by: Thomas Gleixner --- diff --git a/arch/x86/mm/pageattr_64.c b/arch/x86/mm/pageattr_64.c index 8a4f65bf956e..c7b7dfe1d405 100644 --- a/arch/x86/mm/pageattr_64.c +++ b/arch/x86/mm/pageattr_64.c @@ -230,9 +230,14 @@ void global_flush_tlb(void) struct page *pg, *next; struct list_head l; - down_read(&init_mm.mmap_sem); + /* + * Write-protect the semaphore, to exclude two contexts + * doing a list_replace_init() call in parallel and to + * exclude new additions to the deferred_pages list: + */ + down_write(&init_mm.mmap_sem); list_replace_init(&deferred_pages, &l); - up_read(&init_mm.mmap_sem); + up_write(&init_mm.mmap_sem); flush_map(&l);