linux-kernel - Re: Potentially missing "memory" clobbers in bitops.h for x86

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Message-Id: <20190329210918.GZ4102@linux.ibm.com>
Date:   Fri, 29 Mar 2019 14:09:18 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        James Y Knight <jyknight@...gle.com>
Subject: Re: Potentially missing "memory" clobbers in bitops.h for x86

On Fri, Mar 29, 2019 at 01:52:33PM -0700, H. Peter Anvin wrote:
> On 3/29/19 8:54 AM, Alexander Potapenko wrote:
> > 
> >> Of course, this would force the compiler to actually compute the
> >> offset, which would slow things down.  I have no idea whether this
> >> would be better or worse than just using the "memory" clobber.
> > Just adding the "memory" clobber to clear_bit() changes sizes of 5
> > kernel functions (the three mentioned above, plus hub_activate() and
> > native_send_call_func_ipi()) by a small margin.
> > This probably means the performance impact of this clobber is
> > negligible in this case.
> 
> I would agree with that.
> 
> Could you perhaps verify whether or not any of the above functions
> contains a currently manifest bug?
> 
> Note: the atomic versions of these functions obviously need to have
> "volatile" and the clobber anyway, as they are by definition barriers
> and moving memory operations around them would be a very serious error.

The atomic functions that return void don't need to order anything except
the input and output arguments.  The oddness with clear_bit() is that the
memory changed isn't necessarily the quantity referenced by the argument,
if the number of bits specified is large.

So (for example) atomic_inc() does not need a "memory" clobber, right?

							Thanx, Paul