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

On Sat, 29 Sep 2018, Jia-Ju Bai wrote:

quoted

quoted

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).

-- 
Jiri Kosina
SUSE Labs