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With the to-be-fixed commit, the reset_work handler cleared 'host->mrq'
outside of the spinlock protected critical section. That leaves a small
race window during execution of 'tmio_mmc_reset()' where the done_work
handler could grab a pointer to the now invalid 'host->mrq'. Both would
use it to call mmc_request_done() causing problems (see link below).

However, 'host->mrq' cannot simply be cleared earlier inside the
critical section. That would allow new mrqs to come in asynchronously
while the actual reset of the controller still needs to be done. So,
like 'tmio_mmc_set_ios()', an ERR_PTR is used to prevent new mrqs from
coming in but still avoiding concurrency between work handlers.
Reported-by: Dirk Behme <dirk.behme@de.bosch.com>
Closes: https://lore.kernel.org/all/20240220061356.3001761-1-dirk.behme@de.bosch.com/
Fixes: df3ef2d3

 ("mmc: protect the tmio_mmc driver against a theoretical race")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Reviewed-by: Dirk Behme <dirk.behme@de.bosch.com>
Cc: stable@vger.kernel.org # 3.0+
Link: https://lore.kernel.org/r/20240305104423.3177-2-wsa+renesas@sang-engineering.com

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>