On Thu 01-06-23 22:20:38, Zhang Yi wrote: > On 2023/6/1 21:44, Zhihao Cheng wrote: > > 在 2023/6/1 17:41, Jan Kara 写道: > > > > Hi, Jan > >> On Wed 31-05-23 19:50:59, Zhang Yi wrote: > >>> From: Zhihao Cheng <chengzhihao1@xxxxxxxxxx> > >>> > >>> Following process, > >>> > >>> jbd2_journal_commit_transaction > >>> // there are several dirty buffer heads in transaction->t_checkpoint_list > >>> P1 wb_workfn > >>> jbd2_log_do_checkpoint > >>> if (buffer_locked(bh)) // false > >>> __block_write_full_page > >>> trylock_buffer(bh) > >>> test_clear_buffer_dirty(bh) > >>> if (!buffer_dirty(bh)) > >>> __jbd2_journal_remove_checkpoint(jh) > >>> if (buffer_write_io_error(bh)) // false > >>> >> bh IO error occurs << > >>> jbd2_cleanup_journal_tail > >>> __jbd2_update_log_tail > >>> jbd2_write_superblock > >>> // The bh won't be replayed in next mount. > >>> , which could corrupt the ext4 image, fetch a reproducer in [Link]. > >>> > >>> Since writeback process clears buffer dirty after locking buffer head, > >>> we can fix it by checking buffer dirty firstly and then checking buffer > >>> locked, the buffer head can be removed if it is neither dirty nor locked. > >>> > >>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217490 > >>> Fixes: 470decc613ab ("[PATCH] jbd2: initial copy of files from jbd") > >>> Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx> > >>> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx> > >> > >> OK, the analysis is correct but I'm afraid the fix won't be that easy. The > >> reordering of tests you did below doesn't really help because CPU or the > >> compiler are free to order the loads (and stores) in whatever way they > >> wish. You'd have to use memory barriers when reading and modifying bh flags > >> (although the modification side is implicitely handled by the bitlock > >> code) to make this work reliably. But that is IMHO too subtle for this > >> code. > >> > > > > I write two litmus-test files in tools/memory-model to check the memory module > of these two cases as jbd2_log_do_checkpoint() and __cp_buffer_busy() does. <snip litmus tests> > So it looks like the out-of-order situation cannot happen, am I miss something? After thinking about it a bit, indeed CPU cannot reorder the two loads because they are from the same location in memory. Thanks for correcting me on this. I'm not sure whether a C compiler still could not reorder the tests - technically I suspect the C standard does not forbid this although it would have to be really evil compiler to do this. But still I think with the helper function I've suggested things are much more obviously correct. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
