On Mon, 7 Mar 2016, Sedat Dilek wrote:
> Hmm, we are there where I was looking at...
>
> Please, read the reply of Jiri [1], we did some tweaking.
> With CONFIG_FTRACE=n and CONFIG_PROVE_LOCKING=n !
Yes, Jiri was looking more or less at the right place but his
conclusions were wrong.
> *** Part one: ObjectDump of hid-core.o ***
>
> $ objdump -D drivers/hid/usbhid/hid-core.o | awk '/<[^>]*>:$/ { p=0; }
> /<usbhid_close>:/ { p=1; } { if (p) print $0; }' >
> ../objdump-D_hid-core_o_usbhid_close_$(uname -r).txt
This reveals the problem, at last...
> $ cat ../objdump-D_hid-core_o_usbhid_close_4.4.4-1-iniza-small.txt
> 00000000000002e0 <usbhid_close>:
> 2e0: 55 push %rbp
> 2e1: 48 89 e5 mov %rsp,%rbp
> 2e4: 41 57 push %r15
> 2e6: 41 56 push %r14
> 2e8: 41 54 push %r12
> 2ea: 53 push %rbx
> 2eb: 49 89 ff mov %rdi,%r15
> 2ee: 4d 8b b7 e8 1e 00 00 mov 0x1ee8(%r15),%r14
> 2f5: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
> 2fc: 31 f6 xor %esi,%esi
> 2fe: e8 00 00 00 00 callq 303 <usbhid_close+0x23>
mutex_lock(&hid_open_mut);
> 303: 49 8d 9e 88 28 00 00 lea 0x2888(%r14),%rbx
> 30a: 48 89 df mov %rbx,%rdi
> 30d: e8 00 00 00 00 callq 312 <usbhid_close+0x32>
spin_lock_irq(&usbhid->lock);
> 312: 41 ff 8f e4 1d 00 00 decl 0x1de4(%r15)
--hid->open
> 319: 9c pushfq
> 31a: 41 5c pop %r12
> 31c: 48 89 df mov %rbx,%rdi
> 31f: e8 00 00 00 00 callq 324 <usbhid_close+0x44>
> 324: 41 54 push %r12
> 326: 9d popfq
spin_unlock_irq(&usbhid->lock); while attempting to preserve the Z
flag. The problem is that this code sequence will also preserve the
Interrupt Flag!
> 327: 75 23 jne 34c <usbhid_close+0x6c>
if (!--hid->open), testing the Z flag from the decl.
> 329: 4c 89 f7 mov %r14,%rdi
> 32c: e8 3f 00 00 00 callq 370 <hid_cancel_delayed_stuff>
But now hid_cancel_delayed_stuff(usbhid) gets called with interrupts
disabled.
It's hard to call this a compiler bug, but perhaps it is -- I don't
know how programmers are supposed to tell CLANG that a subroutine
modifies the Interrupt Flag in a way that the compiler shouldn't mess
up.
Alan Stern
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