Messages in this thread Patch in this message | | | Date | Thu, 3 Oct 2019 17:12:34 +0100 | From | Mark Rutland <> | Subject | Re: Kernel Concurrency Sanitizer (KCSAN) |
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On Fri, Sep 20, 2019 at 07:51:04PM +0200, Marco Elver wrote: > On Fri, 20 Sep 2019 at 18:47, Dmitry Vyukov <dvyukov@google.com> wrote: > > > > On Fri, Sep 20, 2019 at 6:31 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > On Fri, Sep 20, 2019 at 04:18:57PM +0200, Marco Elver wrote: > > > > We would like to share a new data-race detector for the Linux kernel: > > > > Kernel Concurrency Sanitizer (KCSAN) -- > > > > https://github.com/google/ktsan/wiki/KCSAN (Details: > > > > https://github.com/google/ktsan/blob/kcsan/Documentation/dev-tools/kcsan.rst) > > > > > > Nice! > > > > > > BTW kcsan_atomic_next() is missing a stub definition in <linux/kcsan.h> > > > when !CONFIG_KCSAN: > > > > > > https://github.com/google/ktsan/commit/a22a093a0f0d0b582c82cdbac4f133a3f61d207c#diff-19d7c475b4b92aab8ba440415ab786ec > > > > > > ... and I think the kcsan_{begin,end}_atomic() stubs need to be static > > > inline too. > > Thanks for catching, fixed and pushed. Feel free to rebase your arm64 branch.
Great; I've just done so!
What's the plan for posting a PATCH or RFC series?
The rest of this email is rabbit-holing on the issue KCSAN spotted; sorry about that!
[...]
> > > We have some interesting splats at boot time in stop_machine, which > > > don't seem to have been hit/fixed on x86 yet in the kcsan-with-fixes > > > branch, e.g. > > > > > > [ 0.237939] ================================================================== > > > [ 0.239431] BUG: KCSAN: data-race in multi_cpu_stop+0xa8/0x198 and set_state+0x80/0xb0 > > > [ 0.241189] > > > [ 0.241606] write to 0xffff00001003bd00 of 4 bytes by task 24 on cpu 3: > > > [ 0.243435] set_state+0x80/0xb0 > > > [ 0.244328] multi_cpu_stop+0x16c/0x198 > > > [ 0.245406] cpu_stopper_thread+0x170/0x298 > > > [ 0.246565] smpboot_thread_fn+0x40c/0x560 > > > [ 0.247696] kthread+0x1a8/0x1b0 > > > [ 0.248586] ret_from_fork+0x10/0x18 > > > [ 0.249589] > > > [ 0.250006] read to 0xffff00001003bd00 of 4 bytes by task 14 on cpu 1: > > > [ 0.251804] multi_cpu_stop+0xa8/0x198 > > > [ 0.252851] cpu_stopper_thread+0x170/0x298 > > > [ 0.254008] smpboot_thread_fn+0x40c/0x560 > > > [ 0.255135] kthread+0x1a8/0x1b0 > > > [ 0.256027] ret_from_fork+0x10/0x18 > > > [ 0.257036] > > > [ 0.257449] Reported by Kernel Concurrency Sanitizer on: > > > [ 0.258918] CPU: 1 PID: 14 Comm: migration/1 Not tainted 5.3.0-00007-g67ab35a199f4-dirty #3 > > > [ 0.261241] Hardware name: linux,dummy-virt (DT) > > > [ 0.262517] ==================================================================> > > Thanks, the fixes in -with-fixes were ones I only encountered with > Syzkaller, where I disable KCSAN during boot. I've just added a fix > for this race and pushed to kcsan-with-fixes.
I think that's:
https://github.com/google/ktsan/commit/c1bc8ab013a66919d8347c2392f320feabb14f92
... but that doesn't look quite right to me, as it leaves us with the shape:
do { if (READ_ONCE(msdata->state) != curstate) { curstate = msdata->state; switch (curstate) { ... } ack_state(msdata); } } while (curstate != MULTI_STOP_EXIT);
I don't believe that we have a guarantee of read-after-read ordering between the READ_ONCE(msdata->state) and the subsequent plain access of msdata->state, as we've been caught out on that in the past, e.g.
https://lore.kernel.org/lkml/1506527369-19535-1-git-send-email-will.deacon@arm.com/
... which I think means we could switch on a stale value of msdata->state. That would mean we might handle the same state twice, calling ack_state() more times than expected and corrupting the count.
The compiler could also replace uses of curstate with a reload of msdata->state. If it did so for the while condition, we could skip the expected ack_state() for MULTI_STOP_EXIT, though it looks like that might not matter.
I think we need to make sure that we use a consistent snapshot, something like the below. Assuming I'm not barking up the wrong tree, I can spin this as a proper patch.
Thanks, Mark.
---->8---- diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c index b4f83f7bdf86..67a0b454b5b5 100644 --- a/kernel/stop_machine.c +++ b/kernel/stop_machine.c @@ -167,7 +167,7 @@ static void set_state(struct multi_stop_data *msdata, /* Reset ack counter. */ atomic_set(&msdata->thread_ack, msdata->num_threads); smp_wmb(); - msdata->state = newstate; + WRITE_ONCE(msdata->state, newstate); } /* Last one to ack a state moves to the next state. */ @@ -186,7 +186,7 @@ void __weak stop_machine_yield(const struct cpumask *cpumask) static int multi_cpu_stop(void *data) { struct multi_stop_data *msdata = data; - enum multi_stop_state curstate = MULTI_STOP_NONE; + enum multi_stop_state newstate, curstate = MULTI_STOP_NONE; int cpu = smp_processor_id(), err = 0; const struct cpumask *cpumask; unsigned long flags; @@ -210,8 +210,9 @@ static int multi_cpu_stop(void *data) do { /* Chill out and ensure we re-read multi_stop_state. */ stop_machine_yield(cpumask); - if (msdata->state != curstate) { - curstate = msdata->state; + newstate = READ_ONCE(msdata->state); + if (newstate != curstate) { + curstate = newstate; switch (curstate) { case MULTI_STOP_DISABLE_IRQ: local_irq_disable();
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