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    Subject[PATCH 17/28] ARC: add compiler barrier to LLSC based cmpxchg
    Date
    When auditing cmpxchg call sites, Chuck noted that gcc was optimizing
    away some of the desired LDs.

    | do {
    | new = old = *ipi_data_ptr;
    | new |= 1U << msg;
    | } while (cmpxchg(ipi_data_ptr, old, new) != old);

    was generating to below

    | 8015cef8: ld r2,[r4,0] <-- First LD
    | 8015cefc: bset r1,r2,r1
    |
    | 8015cf00: llock r3,[r4] <-- atomic op
    | 8015cf04: brne r3,r2,8015cf10
    | 8015cf08: scond r1,[r4]
    | 8015cf0c: bnz 8015cf00
    |
    | 8015cf10: brne r3,r2,8015cf00 <-- Branch doesn't go to orig LD

    Although this was fixed by adding a ACCESS_ONCE in this call site, it
    seems safer (for now at least) to add compiler barrier to LLSC based
    cmpxchg

    Reported-by: Chuck Jordan <cjordan@synopsys,com>
    Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
    ---
    arch/arc/include/asm/cmpxchg.h | 9 +++++----
    1 file changed, 5 insertions(+), 4 deletions(-)

    diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
    index 03cd6894855d..90de5c528da2 100644
    --- a/arch/arc/include/asm/cmpxchg.h
    +++ b/arch/arc/include/asm/cmpxchg.h
    @@ -25,10 +25,11 @@ __cmpxchg(volatile void *ptr, unsigned long expected, unsigned long new)
    " scond %3, [%1] \n"
    " bnz 1b \n"
    "2: \n"
    - : "=&r"(prev)
    - : "r"(ptr), "ir"(expected),
    - "r"(new) /* can't be "ir". scond can't take limm for "b" */
    - : "cc");
    + : "=&r"(prev) /* Early clobber, to prevent reg reuse */
    + : "r"(ptr), /* Not "m": llock only supports reg direct addr mode */
    + "ir"(expected),
    + "r"(new) /* can't be "ir". scond can't take LIMM for "b" */
    + : "cc", "memory"); /* so that gcc knows memory is being written here */

    return prev;
    }
    --
    1.9.1


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