Re: [Xen-devel] [PATCH] mini-os: fix bit ops comments and memory clobbers.

[Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>]

Andrew Cooper, le Wed 18 Jun 2014 10:30:03 +0100, a écrit :
> On 18/06/14 09:29, Samuel Thibault wrote:
> > This fixes comments about test_and_clear_bit, set_bit and clear_bit,
> > which are actually not atomic, can be reordered and are not memory
> > barriers.
> >
> > This also drops the empty LOCK and LOCK_PREFIX macros which bring to the
> > confusion.
> >
> > This also adds missing memory clobbers to set_bit and clear_bit.
> >
> > Signed-off-by: Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>
> 
> Do you mean reordered by the compiler or reordered by the pipeline?

Yes.

> The memory clobbers prevent the compiler reordering, but lack of
> [lsm]fence instructions allow the pipeline to reorder the reads/writes
> if it chooses.

Ah, actually I was adding the memory clobbers to tell the compiler that
we are modifying memory, but here we already properly tell so through
"=m". Let me redo the patch without it.

Samuel

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