linux-kernel - Re: [PATCH v4 1/2] introduce test_bit_acquire and use it in wait_on

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Message-ID: <20220802133652.GA27253@willie-the-truck>
Date:   Tue, 2 Aug 2022 14:36:53 +0100
From:   Will Deacon <will@...nel.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] introduce test_bit_acquire and use it in
 wait_on_bit

On Tue, Aug 02, 2022 at 07:38:17AM -0400, Mikulas Patocka wrote:
> 
> 
> On Tue, 2 Aug 2022, Will Deacon wrote:
> 
> > On Mon, Aug 01, 2022 at 12:12:47PM -0400, Mikulas Patocka wrote:
> > > On Mon, 1 Aug 2022, Will Deacon wrote:
> > > > On Mon, Aug 01, 2022 at 06:42:15AM -0400, Mikulas Patocka wrote:
> > > > 
> > > > > Index: linux-2.6/arch/x86/include/asm/bitops.h
> > > > > ===================================================================
> > > > > --- linux-2.6.orig/arch/x86/include/asm/bitops.h	2022-08-01 12:27:43.000000000 +0200
> > > > > +++ linux-2.6/arch/x86/include/asm/bitops.h	2022-08-01 12:27:43.000000000 +0200
> > > > > @@ -203,8 +203,10 @@ arch_test_and_change_bit(long nr, volati
> > > > >  
> > > > >  static __always_inline bool constant_test_bit(long nr, const volatile unsigned long *addr)
> > > > >  {
> > > > > -	return ((1UL << (nr & (BITS_PER_LONG-1))) &
> > > > > +	bool r = ((1UL << (nr & (BITS_PER_LONG-1))) &
> > > > >  		(addr[nr >> _BITOPS_LONG_SHIFT])) != 0;
> > > > > +	barrier();
> > > > > +	return r;
> > > > 
> > > > Hmm, I find it a bit weird to have a barrier() here given that 'addr' is
> > > > volatile and we don't need a barrier() like this in the definition of
> > > > READ_ONCE(), for example.
> > > 
> > > gcc doesn't reorder two volatile accesses, but it can reorder non-volatile
> > > accesses around volatile accesses.
> > > 
> > > The purpose of the compiler barrier is to make sure that the non-volatile 
> > > accesses that follow test_bit are not reordered by the compiler before the 
> > > volatile access to addr.
> > 
> > If we need these accesses to be ordered reliably, then we need a CPU barrier
> > and that will additionally prevent the compiler reordering. So I still don't
> > think we need the barrier() here.
> 
> This is x86-specific code. x86 has strong memory ordering, so we only care 
> about compiler reordering.

Indeed, but what I'm trying to say is that the _caller_ would have a memory
barrier in this case, and so there's no need for one in here. test_bit() does
not have ordering semantics.

Will