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    SubjectRe: [PATCH] dm.c - device-mapper I/O path fixes
    Date
    On Wednesday 11 December 2002 13:24, Denis Vlasenko wrote:
    > On 11 December 2002 12:18, Joe Thornber wrote:
    > > On Wed, Dec 11, 2002 at 07:16:53AM -0600, Kevin Corry wrote:
    > > > However, it might be a good idea to consider how bio's keep track
    > > > of errors. When a bio is created, it is marked UPTODATE. Then, if
    > > > any part of a bio takes an error, the UPTODATE flag is turned off.
    > > > When the whole bio completes, if the UPTODATE flag is still on,
    > > > there were no errors during the i/o. Perhaps the "error" field in
    > > > "struct dm_io" could be modified to use this method of error
    > > > tracking? Then we could change dec_pending() to be something like:
    > > >
    > > > if (error)
    > > > clear_bit(DM_IO_UPTODATE, &io->error);
    > > >
    > > > with a "set_bit(DM_IO_UPTODATE, &ci.io->error);" in __bio_split().
    > >
    > > The problem with this is you don't keep track of the specific error
    > > to later pass to bio_endio(io->bio...). I guess it all comes down to
    > > just how expensive that spin lock is; and since locking only occurs
    > > when there's an error I'm happy with things as they are.
    >
    > lock();
    > a = b;
    > unlock();
    >
    > Store of ints is atomic anyway. You need locking if a is a larger entity,
    > say, a struct.

    Storing an int is *not* atomic unless it is declared as atomic_t and you use
    the appropriate macros (see include/asm-*/atomic.h). Remember, we are talking
    about a field in a data structure that can be accessed from multiple threads
    on multiple CPUs.

    --
    Kevin Corry
    corryk@us.ibm.com
    http://evms.sourceforge.net/
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