Messages in this thread | | | From | Dmitry Vyukov <> | Date | Thu, 17 Sep 2015 20:38:15 +0200 | Subject | Re: [PATCH] kernel: fix data race in put_pid |
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On Thu, Sep 17, 2015 at 8:09 PM, Oleg Nesterov <oleg@redhat.com> wrote: > On 09/17, Dmitry Vyukov wrote: >> >> I can update the patch description, but let me explain it here first. > > Yes thanks. > >> Here is the essence of what happens: > > Aha, so you really meant that 2 put_pid's can race with each other, > >> // thread 1 >> 1: pid->foo = 1; // foo is the first word of pid object >> // then it does put_pid >> 2: atomic_dec_and_test(&pid->count) // decrements count to 1 and >> returns false so the function returns >> >> // thread 2 >> // executes put_pid >> 3: atomic_load(&pid->count); // returns 1, so proceed to kmem_cache_free >> // then kmem_cache_free does: >> 5: head->freelist = (void*)pid; >> >> This can be executed as: >> >> 4: *(void**)pid = head->freelist; >> 1: pid->foo = 1; // foo is the first word of pid object >> 2: atomic_dec_and_test(&pid->count) // decrements count to 1 and >> returns false so the function returns >> 3: atomic_load(&pid->count); // returns 1, so proceed to kmem_cache_free >> 5: head->freelist = (void*)pid; > > Unless I am totally confused, everything is simpler. We can forget > about the hoisting, freelist, etc. > > Thread 2 can see the result of atomic_dec_and_test(), but not the > result of "pid->foo = 1". In this case in can free the object which > can be re-allocated _before_ STORE(pid->foo) completes. Of course, > this would be really bad.
Yes, that's what I mean. A missed memory barrier can break in lots of magical, complex ways. So I generally prefer to not think about concrete scenarios. Passing a non-acquired object to kfree is a data race.
> I need to recheck, but afaics this is not possible. This optimization > is fine, but probably needs a comment. We rely on delayed_put_pid() > called by RCU. And note that nobody can write to this pid after it > is removed from the rcu-protected list. > > So I think this is false alarm, but I'll try to recheck tomorrow, it > is too late for me today.
Well, if that would be true, then put_pid would not contain any atomic operations. Here is the report from KTSAN that I observe:
ThreadSanitizer: data-race in kt_memblock_free
Write of size 8 by thread T107 (K630): [<ffffffff812499ef>] kt_memblock_free+0xdf/0x150 mm/ktsan/memblock.c:90 [<ffffffff81249704>] ktsan_memblock_free+0xc4/0xf0 mm/ktsan/ktsan.c:251 (discriminator 6) [< inlined >] kmem_cache_free+0x99/0x610 __cache_free mm/slab.c:3383 [<ffffffff81239149>] kmem_cache_free+0x99/0x610 mm/slab.c:3561 [<ffffffff810b4095>] put_pid+0x85/0xa0 kernel/pid.c:247 [<ffffffff810b40ce>] delayed_put_pid+0x1e/0x30 kernel/pid.c:256 [< inlined >] rcu_process_callbacks+0x410/0xa70 __rcu_reclaim kernel/rcu/rcu.h:118 [< inlined >] rcu_process_callbacks+0x410/0xa70 rcu_do_batch kernel/rcu/tree.c:2669 [< inlined >] rcu_process_callbacks+0x410/0xa70 invoke_rcu_callbacks kernel/rcu/tree.c:2937 [< inlined >] rcu_process_callbacks+0x410/0xa70 __rcu_process_callbacks kernel/rcu/tree.c:2904 [<ffffffff811044d0>] rcu_process_callbacks+0x410/0xa70 kernel/rcu/tree.c:2921 [<ffffffff8108f18d>] __do_softirq+0xad/0x2d0 kernel/softirq.c:273 [< inlined >] irq_exit+0x98/0xa0 invoke_softirq kernel/softirq.c:350 [<ffffffff8108f528>] irq_exit+0x98/0xa0 kernel/softirq.c:391 [< inlined >] smp_apic_timer_interrupt+0x63/0x80 exiting_irq ./arch/x86/include/asm/apic.h:655 [<ffffffff8105cd33>] smp_apic_timer_interrupt+0x63/0x80 arch/x86/kernel/apic/apic.c:915 [<ffffffff81e9661b>] apic_timer_interrupt+0x6b/0x70 arch/x86/entry/entry_64.S:782 [< inlined >] complete+0x41/0x50 spin_unlock_irqrestore include/linux/spinlock.h:372 [<ffffffff810d90b1>] complete+0x41/0x50 kernel/sched/completion.c:36 [<ffffffff810b7f11>] kthread+0x131/0x180 kernel/kthread.c:200 [<ffffffff81e95bdf>] ret_from_fork+0x3f/0x70 arch/x86/entry/entry_64.S:526 DBG: cpu = ffff88063fc1fe68 DBG: cpu id = 0
Previous read of size 8 by thread T28 (K25): [<ffffffff810b4043>] put_pid+0x33/0xa0 kernel/pid.c:244 [<ffffffff810874a8>] _do_fork+0x1a8/0x550 kernel/fork.c:1746 [<ffffffff8108788c>] kernel_thread+0x3c/0x60 kernel/fork.c:1772 [<ffffffff810a7e1c>] __call_usermodehelper+0x5c/0x90 kernel/kmod.c:317 [<ffffffff810aebed>] process_one_work+0x2ad/0x6f0 kernel/workqueue.c:2036 [<ffffffff810af769>] worker_thread+0xb9/0x730 kernel/workqueue.c:2170 [<ffffffff810b7f41>] kthread+0x161/0x180 kernel/kthread.c:207 [<ffffffff81e95bdf>] ret_from_fork+0x3f/0x70 arch/x86/entry/entry_64.S:526
Once put_pid indeed comes from rcu_process_callbacks, but another comes from _do_fork and it is not in an rcu read critical section.
-- Dmitry Vyukov, Software Engineer, dvyukov@google.com Google Germany GmbH, Dienerstraße 12, 80331, München Geschäftsführer: Graham Law, Christine Elizabeth Flores Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg Diese E-Mail ist vertraulich. Wenn Sie nicht der richtige Adressat sind, leiten Sie diese bitte nicht weiter, informieren Sie den Absender und löschen Sie die E-Mail und alle Anhänge. Vielen Dank. This e-mail is confidential. If you are not the right addressee please do not forward it, please inform the sender, and please erase this e-mail including any attachments. Thanks.
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