Re: [PATCH V2] hid: hid-core: Fix a sleep-in-atomic-context bug in __hid_request()

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On 2018/9/30 3:20, Jiri Kosina wrote:
On Sat, 29 Sep 2018, Jia-Ju Bai wrote:

picolcd_send_and_wait (acquire a spinlock)
    hid_hw_request
      __hid_request
        hid_alloc_report_buf(GFP_KERNEL)

picolcd_reset (acquire a spinlock)
    hid_hw_request
      __hid_request
        hid_alloc_report_buf(GFP_KERNEL)

lg4ff_play (acquire a spinlock)
    hid_hw_request
      __hid_request
        hid_alloc_report_buf(GFP_KERNEL)

lg4ff_set_autocenter_ffex (acquire a spinlock)
    hid_hw_request
      __hid_request
        hid_alloc_report_buf(GFP_KERNEL)
Hm, so it's always drivers calling out into core in atomic context. So
either we take this, and put our bets on being able to allocate the buffer
without sleeping,
In my opinion, I prefer this way.
Why? Forcing all the report buffer to be limited to be non-sleeping
allocations just because of two drivers, looks like an overkill, and
actually calls for more issues (as GFP_ATOMIC is of course in principle
less likely to succeed).


Okay, I thought that using GFP_ATOMIC is the simplest way to fix these bugs.
But I check the Linux kernel code again, and find that hid_hw_request() are called at many places.
So changing this function may affect many drivers.
I agree to only change the two drivers, and explicitly anotate __hid_request() with might_sleep().


Best wishes,
Jia-Ju Bai



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