On Tue, Aug 23, 2022 at 12:18:47PM +1000, Dave Chinner wrote: > On Mon, Aug 22, 2022 at 05:33:19PM -0700, Darrick J. Wong wrote: > > On Wed, Aug 10, 2022 at 09:03:48AM +1000, Dave Chinner wrote: > > > From: Dave Chinner <dchinner@xxxxxxxxxx> > > > > > > Whenever we write an iclog, we call xlog_assign_tail_lsn() to update > > > the current tail before we write it into the iclog header. This > > > means we have to take the AIL lock on every iclog write just to > > > check if the tail of the log has moved. > > > > > > This doesn't avoid races with log tail updates - the log tail could > > > move immediately after we assign the tail to the iclog header and > > > hence by the time the iclog reaches stable storage the tail LSN has > > > moved forward in memory. Hence the log tail LSN in the iclog header > > > is really just a point in time snapshot of the current state of the > > > AIL. > > > > > > With this in mind, if we simply update the in memory log->l_tail_lsn > > > every time it changes in the AIL, there is no need to update the in > > > memory value when we are writing it into an iclog - it will already > > > be up-to-date in memory and checking the AIL again will not change > > > this. > > > > This is too subtle for me to understand -- does the codebase > > already update l_tail_lsn? Does this patch make it do that? > > tl;dr: if the AIL is empty, log->l_tail_lsn is not updated on the > first insert of a new item into the AILi and hence is stale. > xlog_state_release_iclog() currently works around that by calling > xlog_assign_tail_lsn() to get the tail lsn from the AIL. This change > makes sure log->l_tail_lsn is always up to date. > > In more detail: > > The tail update occurs in xfs_ail_update_finish(), but only if we > pass in a non-zero tail_lsn. xfs_trans_ail_update_bulk() will only > set a non-zero tail_lsn if it moves the log item at the tail of the > log (i.e. we relog the tail item and move it forwards in the AIL). > > Hence if we pass a non-zero tail_lsn to xfs_ail_update_finish(), it > indicates it needs to check it against the LSN of the item currently > at the tail of the AIL. If the tail LSN has not changed, we do > nothing, if it has changed, then we call > xlog_assign_tail_lsn_locked() to update the log tail. > > The problem with the current code is that if the AIL is empty when > we insert the first item, we've actually moved the log tail but we > do not update the log tail (i.e. tail_lsn is zero in this case). If > we then release an iclog for writing at this point in time, the tail > lsn it writes into the iclog header would be wrong - it does not > reflect the log tail as defined by the AIL and the checkpoint that > has just been committed. > > Hence xlog_state_release_iclog() called xlog_assign_tail_lsn() to > ensure that it checked that the tail LSN it applies to the iclog > reflects the current state of the AIL. i.e. it checks if there is an > item in the AIL, and if so, grabs the tail_lsn from the AIL. This > works around the fact the AIL doesn't update the log tail on the > first insert. > > Hence what this patch does is have xfs_trans_ail_update_bulk set > the tail_lsn passed to xfs_ail_update_finish() to NULLCOMMITLSN when > it does the first insert into the AIL. NULLCOMMITLSN is a > non-zero value that won't match with the LSN of items we just > inserted into the AIL, and hence xfs_ail_update_finish() will go an > update the log tail in this case. > > Hence we close the hole when the log->l_tail_lsn is incorrect after > the first insert into the AIL, and hence we no longer need to update > the log->l_tail_lsn when reading it into the iclog header - > log->l_tail_lsn is always up to date, and so we can now just read it > in xlog_state_release_iclog() rather than having to grab the AIL > lock and checking the AIL to update log->l_tail_lsn with the correct > tail value from iclog IO submission.... Ahhh, ok, I get it now. Thanks for the explanation. --D > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx
