Messages in this thread | | | Subject | Re: [PATCH 1/8] scsi: Drop struct Scsi_Host->host_lock around SHT->queuecommand() | From | Tim Chen <> | Date | Fri, 17 Sep 2010 10:49:59 -0700 |
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On Fri, 2010-09-17 at 09:41 -0700, Nicholas A. Bellinger wrote:
> > > > > > What I was actually thinking of for the atomic is that we'd let it range > > > [1..INT_MAX] so a zero was an indicator of no use of this. Then the > > > actual code could become > > > > > > if (atomic_read(x)) { > > > do { > > > y = atomic_add_return(1, x); > > > } while (y == 0); > > > } > > > > The conversion of struct scsi_cmnd->serial_number to atomic_t and the > > above code for scsi_cmd_get_serial() sounds perfectly reasonable to me. > > > > Actually, that should be the conversion of struct > Scsi_Host->cmd_serial_number to an atomic_t. AFAICT there is no reason > for struct scsi_cmnd->serial_number needing to be an atomic_t.
Just want to verify the hidden assumption we have here when the atomic int Scsi_Host->cmd_serial_number counter overflow after increment. The counter itself then becomes negative. We are assuming that when we do type conversion back to unsigned long scsi_cmnd->serial_number, we will get the right thing.
So for 32-bit int, we expect if we start with 0x7fffffff in hex and the expected sequence will be
2147483647 (int) -> 2147483647 (unsigned long) [0x7fffffff] +1 -2147483648 (int) -> 2147483648 (unsigned long) [0x80000000] +1 -2147483647 (int) -> 2147483649 (unsigned long) [0x80000001] If there is architecture where the above assumption is not true (which I'm not aware of but just checking), then we should manually wrap the atomic counter to 1 when counter overflow.
Tim
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