Messages in this thread | | | Date | Fri, 6 Oct 2023 16:39:54 +0200 | Subject | Re: [PATCH v1 1/1] xarray: fix the data-race in xas_find_chunk() by using READ_ONCE() | From | Mirsad Todorovac <> |
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On 9/19/2023 6:20 AM, Matthew Wilcox wrote: > On Mon, Sep 18, 2023 at 11:56:36AM -0700, Yury Norov wrote: >> Guys, I lost the track of the conversation. In the other email Mirsad >> said: >> Which was the basic reason in the first place for all this, because something changed >> data from underneath our fingers .. >> >> It sounds clearly to me that this is a bug in xarray, *revealed* by >> find_next_bit() function. But later in discussion you're trying to 'fix' >> find_*_bit(), like if find_bit() corrupted the bitmap, but it's not. > > No, you're really confused. That happens. > > KCSAN is looking for concurrency bugs. That is, does another thread > mutate the data "while" we're reading it. It does that by reading > the data, delaying for a few instructions and reading it again. If it > changed, clearly there's a race. That does not mean there's a bug! > > Some races are innocuous. Many races are innocuous! The problem is > that compilers sometimes get overly clever and don't do the obvious > thing you ask them to do. READ_ONCE() serves two functions here; > one is that it tells the compiler not to try anything fancy, and > the other is that it tells KCSAN to not bother instrumenting this > load; no load-delay-reload. > >> In previous email Jan said: >> for any sane compiler the generated assembly with & without READ_ONCE() >> will be exactly the same. >> >> If the code generated with and without READ_ONCE() is the same, the >> behavior would be the same, right? If you see the difference, the code >> should differ. > > Hopefully now you understand why this argument is wrong ... > >> You say that READ_ONCE() in find_bit() 'fixes' 200 KCSAN BUG warnings. To >> me it sounds like hiding the problems instead of fixing. If there's a race >> between writing and reading bitmaps, it should be fixed properly by >> adding an appropriate serialization mechanism. Shutting Kcsan up with >> READ_ONCE() here and there is exactly the opposite path to the right direction. > > Counterpoint: generally bitmaps are modified with set_bit() which > actually is atomic. We define so many bitmap things as being atomic > already, it doesn't feel like making find_bit() "must be protected" > as a useful use of time. > > But hey, maybe I'm wrong. Mirsad, can you send Yury the bug reports > for find_bit and friends, and Yury can take the time to dig through them > and see if there are any real races in that mess? > >> Every READ_ONCE must be paired with WRITE_ONCE, just like atomic >> reads/writes or spin locks/unlocks. Having that in mind, adding >> READ_ONCE() in find_bit() requires adding it to every bitmap function >> out there. And this is, as I said before, would be an overhead for >> most users. > > I don't believe you. Telling the compiler to stop trying to be clever > rarely results in a performance loss.
Hi Mr. Wilcox,
Do you think we should submit a formal patch for this data-race?
Thank you.
Best regards, Mirsad Todorovac
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