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SubjectRE: [PATCH v7 04/21] x86/split_lock: Align x86_capability to unsigned long to avoid split locked access
Date
From: David Laight
> Sent: 18 April 2019 10:21
> From: Fenghua Yu
> > Sent: 17 April 2019 22:34
> >
> > set_cpu_cap() calls locked BTS and clear_cpu_cap() calls locked BTR to
> > operate on bitmap defined in x86_capability.
> >
> > Locked BTS/BTR accesses a single unsigned long location. In 64-bit mode,
> > the location is at:
> > base address of x86_capability + (bit offset in x86_capability / 64) * 8
> >
> > Since base address of x86_capability may not be aligned to unsigned long,
> > the single unsigned long location may cross two cache lines and
> > accessing the location by locked BTS/BTR introductions will cause
> > split lock.
>
> Isn't the problem that the type (and definition) of x86_capability[] are wrong.
> If the 'bitmap' functions are used for it, it should be defined as a bitmap.
> This would make it 'unsigned long' not __u32.
>
> This type munging of bitmaps only works on LE systems.
>
> OTOH the locked BTS/BTR instructions could be changed to use 32 bit accesses.
> ISTR some of the associated functions use byte accesses.
>
> Perhaps there ought to be asm wrappers for BTS/BTR that do 8bit and
> 32bit accesses.

A quick look shows that this isn't the only __32[] that is being
cast to (unsigned long) and then to set/test/clear_bit() in those
files.

I wonder how much other code is applying such casts?

David

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