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Mon, 02 Mar 2020 05:42:41 -0800 (PST) Date: Mon, 2 Mar 2020 13:42:40 +0000 From: Wei Yang To: Qian Cai Cc: akpm@linux-foundation.org, elver@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/swapfile: fix data races in try_to_unuse() Message-ID: <20200302134240.6i32e4qmgvqiztz2@master> Reply-To: Wei Yang References: <1582578903-29294-1-git-send-email-cai@lca.pw> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1582578903-29294-1-git-send-email-cai@lca.pw> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 24, 2020 at 04:15:03PM -0500, Qian Cai wrote: >si->inuse_pages could be accessed concurrently as noticed by KCSAN, > > write to 0xffff98b00ebd04dc of 4 bytes by task 82262 on cpu 92: > swap_range_free+0xbe/0x230 > swap_range_free at mm/swapfile.c:719 > swapcache_free_entries+0x1be/0x250 > free_swap_slot+0x1c8/0x220 > __swap_entry_free.constprop.19+0xa3/0xb0 > free_swap_and_cache+0x53/0xa0 > unmap_page_range+0x7e0/0x1ce0 > unmap_single_vma+0xcd/0x170 > unmap_vmas+0x18b/0x220 > exit_mmap+0xee/0x220 > mmput+0xe7/0x240 > do_exit+0x598/0xfd0 > do_group_exit+0x8b/0x180 > get_signal+0x293/0x13d0 > do_signal+0x37/0x5d0 > prepare_exit_to_usermode+0x1b7/0x2c0 > ret_from_intr+0x32/0x42 > > read to 0xffff98b00ebd04dc of 4 bytes by task 82499 on cpu 46: > try_to_unuse+0x86b/0xc80 > try_to_unuse at mm/swapfile.c:2185 > __x64_sys_swapoff+0x372/0xd40 > do_syscall_64+0x91/0xb05 > entry_SYSCALL_64_after_hwframe+0x49/0xbe > >The plain reads in try_to_unuse() are outside si->lock critical section >which result in data races that could be dangerous to be used in a loop. >Fix them by adding READ_ONCE(). > >Signed-off-by: Qian Cai >--- > mm/swapfile.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > >diff --git a/mm/swapfile.c b/mm/swapfile.c >index a65622eec66f..36fd1536a83d 100644 >--- a/mm/swapfile.c >+++ b/mm/swapfile.c >@@ -2137,7 +2137,7 @@ int try_to_unuse(unsigned int type, bool frontswap, > swp_entry_t entry; > unsigned int i; > >- if (!si->inuse_pages) >+ if (!READ_ONCE(si->inuse_pages)) > return 0; > > if (!frontswap) >@@ -2153,7 +2153,7 @@ int try_to_unuse(unsigned int type, bool frontswap, > > spin_lock(&mmlist_lock); > p = &init_mm.mmlist; >- while (si->inuse_pages && >+ while (READ_ONCE(si->inuse_pages) && The change is not wrong. But since it is not protected by the lock, some status in swap_info_struct could still be modified after we test this inuse_pages is not zero. Would this be some problem? -- Wei Yang Help you, Help me