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Use WRITE_ONCE() for the lockless flag update. The + * ordered MMIO accessor used to ring the doorbell keeps this store + * visible before the platform is notified. + */ + if (pchan->plat_irq > 0) + WRITE_ONCE(pchan->chan_in_use, true); ret = pcc_chan_reg_read_modify_write(&pchan->db); if (!ret && pchan->plat_irq > 0) - pchan->chan_in_use = true; + WRITE_ONCE(pchan->chan_in_use, false); return ret; } -- 2.43.0 iAt the end of the above code, WRITE_ONCE(pchan->chan_in_use, false); should be WRITE_ONCE(pchan->chan_in_use, true); In order to keep the original semantics. The flag is cleared when the messages is ACKed, not here. This version causes a hang.