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    Subject[PATCH AUTOSEL 4.20 047/117] powerpc/xmon: Fix invocation inside lock region
    Date
    From: Breno Leitao <leitao@debian.org>

    [ Upstream commit 8d4a862276a9c30a269d368d324fb56529e6d5fd ]

    Currently xmon needs to get devtree_lock (through rtas_token()) during its
    invocation (at crash time). If there is a crash while devtree_lock is being
    held, then xmon tries to get the lock but spins forever and never get into
    the interactive debugger, as in the following case:

    int *ptr = NULL;
    raw_spin_lock_irqsave(&devtree_lock, flags);
    *ptr = 0xdeadbeef;

    This patch avoids calling rtas_token(), thus trying to get the same lock,
    at crash time. This new mechanism proposes getting the token at
    initialization time (xmon_init()) and just consuming it at crash time.

    This would allow xmon to be possible invoked independent of devtree_lock
    being held or not.

    Signed-off-by: Breno Leitao <leitao@debian.org>
    Reviewed-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/powerpc/xmon/xmon.c | 18 ++++++++++++++----
    1 file changed, 14 insertions(+), 4 deletions(-)

    diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
    index 36b8dc47a3c3..b566203d09c5 100644
    --- a/arch/powerpc/xmon/xmon.c
    +++ b/arch/powerpc/xmon/xmon.c
    @@ -75,6 +75,9 @@ static int xmon_gate;
    #define xmon_owner 0
    #endif /* CONFIG_SMP */

    +#ifdef CONFIG_PPC_PSERIES
    +static int set_indicator_token = RTAS_UNKNOWN_SERVICE;
    +#endif
    static unsigned long in_xmon __read_mostly = 0;
    static int xmon_on = IS_ENABLED(CONFIG_XMON_DEFAULT);

    @@ -358,7 +361,6 @@ static inline void disable_surveillance(void)
    #ifdef CONFIG_PPC_PSERIES
    /* Since this can't be a module, args should end up below 4GB. */
    static struct rtas_args args;
    - int token;

    /*
    * At this point we have got all the cpus we can into
    @@ -367,11 +369,11 @@ static inline void disable_surveillance(void)
    * If we did try to take rtas.lock there would be a
    * real possibility of deadlock.
    */
    - token = rtas_token("set-indicator");
    - if (token == RTAS_UNKNOWN_SERVICE)
    + if (set_indicator_token == RTAS_UNKNOWN_SERVICE)
    return;

    - rtas_call_unlocked(&args, token, 3, 1, NULL, SURVEILLANCE_TOKEN, 0, 0);
    + rtas_call_unlocked(&args, set_indicator_token, 3, 1, NULL,
    + SURVEILLANCE_TOKEN, 0, 0);

    #endif /* CONFIG_PPC_PSERIES */
    }
    @@ -3688,6 +3690,14 @@ static void xmon_init(int enable)
    __debugger_iabr_match = xmon_iabr_match;
    __debugger_break_match = xmon_break_match;
    __debugger_fault_handler = xmon_fault_handler;
    +
    +#ifdef CONFIG_PPC_PSERIES
    + /*
    + * Get the token here to avoid trying to get a lock
    + * during the crash, causing a deadlock.
    + */
    + set_indicator_token = rtas_token("set-indicator");
    +#endif
    } else {
    __debugger = NULL;
    __debugger_ipi = NULL;
    --
    2.19.1
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