On thu, 24 Oct 2013 19:32:15 +0800, Wang Shilong wrote: > On 10/24/2013 06:08 PM, Chris Mason wrote: >> Quoting Stefan Behrens (2013-10-23 13:21:34) >>> On Tue, 22 Oct 2013 18:55:59 +0200, Bob Marley wrote: >>>> On 22/10/2013 10:37, Stefan Behrens wrote: >>>>> I don't believe that this issue can ever happen. I don't believe that >>>>> somewhere on the path to the flash memory, to the magnetic disc or to >>>>> the drive's cache memory, someone interrupts a 4KB write in the middle >>>>> of operation to read from this 4KB area. This is not an issue IMHO. >>>> I think I have read that unfortunately it can happen. >>>> SAS and SATA specs for disks do not mandate that if a write is in-flight >>>> but still not completed, reads from the same sector should return the >>>> value it is being written; they can return the old value. >>>> I also think that Linux does not check either. >>> If the _old_ 4KB block is returned, that's fine and won't cause a >>> checksum error. >>> >>> The patch in question addresses the case that Btrfs submits a write >>> request for a 4KB block, and a concurrent read request for that 4KB >>> block reads partially the old block and partially the new block, >>> resulting in a checksum error reported in the scrub statistic counters. >> Concurrent reads and writes to the device are completely undefined, and >> Any combination of old, new, random memory corruption wouldn't >> surprise me...I'd rather avoid them ;) >> >> Doing the transaction join during the super read is probably the least >> complex choice. > Yeah, by joining transaction we can solve this problem, but it is a little > confused, > because we don't involve writting in scrubing supers. > > And the only race condition happens in commiting transaction, Miao also > pointed out that > maybe the best way is to move btrfs_scrub_continue after write_ctree_super().
Sorry, My miss. btrfs_scrub_continue() is behind write_ctree_super() all the while, so the above problem doesn't exist. Thanks Miao > > Thanks, > Wang >> -chris >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in >> the body of a message to [email protected] >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> > > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to [email protected] > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
