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    Subject[PATCH 4.9 081/128] kgdb: Fix spurious true from in_dbg_master()
    Date
    From: Daniel Thompson <daniel.thompson@linaro.org>

    [ Upstream commit 3fec4aecb311995189217e64d725cfe84a568de3 ]

    Currently there is a small window where a badly timed migration could
    cause in_dbg_master() to spuriously return true. Specifically if we
    migrate to a new core after reading the processor id and the previous
    core takes a breakpoint then we will evaluate true if we read
    kgdb_active before we get the IPI to bring us to halt.

    Fix this by checking irqs_disabled() first. Interrupts are always
    disabled when we are executing the kgdb trap so this is an acceptable
    prerequisite. This also allows us to replace raw_smp_processor_id()
    with smp_processor_id() since the short circuit logic will prevent
    warnings from PREEMPT_DEBUG.

    Fixes: dcc7871128e9 ("kgdb: core changes to support kdb")
    Suggested-by: Will Deacon <will@kernel.org>
    Link: https://lore.kernel.org/r/20200506164223.2875760-1-daniel.thompson@linaro.org
    Reviewed-by: Douglas Anderson <dianders@chromium.org>
    Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    include/linux/kgdb.h | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
    index e465bb15912d..6be5545d3584 100644
    --- a/include/linux/kgdb.h
    +++ b/include/linux/kgdb.h
    @@ -317,7 +317,7 @@ extern void gdbstub_exit(int status);
    extern int kgdb_single_step;
    extern atomic_t kgdb_active;
    #define in_dbg_master() \
    - (raw_smp_processor_id() == atomic_read(&kgdb_active))
    + (irqs_disabled() && (smp_processor_id() == atomic_read(&kgdb_active)))
    extern bool dbg_is_early;
    extern void __init dbg_late_init(void);
    #else /* ! CONFIG_KGDB */
    --
    2.25.1


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