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    SubjectRe: [PATCH v4 1/2] introduce test_bit_acquire and use it in wait_on_bit


    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.

    > > Index: linux-2.6/include/linux/wait_bit.h
    > > ===================================================================
    > > --- linux-2.6.orig/include/linux/wait_bit.h 2022-08-01 12:27:43.000000000 +0200
    > > +++ linux-2.6/include/linux/wait_bit.h 2022-08-01 12:27:43.000000000 +0200
    > > @@ -71,7 +71,7 @@ static inline int
    > > wait_on_bit(unsigned long *word, int bit, unsigned mode)
    > > {
    > > might_sleep();
    > > - if (!test_bit(bit, word))
    > > + if (!test_bit_acquire(bit, word))
    > > return 0;
    > > return out_of_line_wait_on_bit(word, bit,
    > > bit_wait,
    >
    > Yet another approach here would be to leave test_bit as-is and add a call to
    > smp_acquire__after_ctrl_dep() since that exists already -- I don't have
    > strong opinions about it, but it saves you having to add another stub to
    > x86.

    It would be the same as my previous patch with smp_rmb() that Linus didn't
    like. But I think smp_rmb (or smp_acquire__after_ctrl_dep) would be
    correct here.

    > Will

    Mikulas

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