Messages in this thread | | | Subject | Re: [PATCH linux-next] kernel/fork.c: annotate data races for copy_process | From | Zefan Li <> | Date | Wed, 17 Jun 2020 17:19:35 +0800 |
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On 2020/6/17 17:08, Chenweilong wrote: >> On Tue, Jun 09, 2020 at 11:08:01AM +0800, Weilong Chen wrote: >>> The check is only there to stop root fork bombs. >>> >>> BUG: KCSAN: data-race in copy_process / copy_process >>> >>> write to 0xffffffff86f87d20 of 4 bytes by task 7121 on cpu 5: >>> copy_process+0x2e1a/0x3af0 kernel/fork.c:2285 >>> _do_fork+0xf7/0x790 kernel/fork.c:2430 >>> __do_sys_clone+0xf9/0x130 kernel/fork.c:2585 __se_sys_clone >>> kernel/fork.c:2566 [inline] >>> __x64_sys_clone+0x6c/0x80 kernel/fork.c:2566 >>> do_syscall_64+0xc7/0x3b0 arch/x86/entry/common.c:295 >>> entry_SYSCALL_64_after_hwframe+0x44/0xa9 >>> >>> read to 0xffffffff86f87d20 of 4 bytes by task 7125 on cpu 3: >>> copy_process+0x9eb/0x3af0 kernel/fork.c:1967 >>> _do_fork+0xf7/0x790 kernel/fork.c:2430 >>> __do_sys_clone+0xf9/0x130 kernel/fork.c:2585 __se_sys_clone >>> kernel/fork.c:2566 [inline] >>> __x64_sys_clone+0x6c/0x80 kernel/fork.c:2566 >>> do_syscall_64+0xc7/0x3b0 arch/x86/entry/common.c:295 >>> entry_SYSCALL_64_after_hwframe+0x44/0xa9 >>> >>> Signed-off-by: Weilong Chen <chenweilong@huawei.com> >> >> Plumbing data_race() in there just to taper over this seems ugly. >> Before we do that we should probably simply make nr_threads atomic_t. > Will using atomic_t cause performance degradation ? I don’t know why atomic was not used in the beginning. > >> Also, where's the link to the syzbot/kcsan report? Or did you get this report from somewhere else? > I got this from local test.
There is a comment just above the if statement to explain this race:
/* * If multiple threads are within copy_process(), then this check * triggers too late. This doesn't hurt, the check is only there * to stop root fork bombs. */
This race won't go away by making nr_threads atomic, because I think it is tasklist_lock that protects nr_thread.
Adding data_race() here I think makes the code more readable, as a supplementary to the code comment.
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