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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. >> > Did you run and seeing hang or just code inspection. If latter, have you > considered that modified code sets it true before doorbell is rung and > set to false only if there is a failure to ring the doorbell ? > Yes, I saw the hang, and debugged to see this line semantically different.  I changed it to true and things resumed working.  !ret is 0  so that happens in the success case.