On Wed 2020-06-10 12:24:01, John Ogness wrote: > On 2020-06-10, Petr Mladek <[email protected]> wrote: > >> >> --- /dev/null > >> >> +++ b/kernel/printk/printk_ringbuffer.c > >> >> +/* > >> >> + * Given a data ring (text or dict), put the associated descriptor of > >> >> each > >> >> + * data block from @lpos_begin until @lpos_end into the reusable state. > >> >> + * > >>>> + * If there is any problem making the associated descriptor reusable, > >>>> either > >>>> + * the descriptor has not yet been committed or another writer task has > >>>> + * already pushed the tail lpos past the problematic data block. > >>>> Regardless, > >>>> + * on error the caller can re-load the tail lpos to determine the > >>>> situation. > >>>> + */ > >>>> +static bool data_make_reusable(struct printk_ringbuffer *rb, > >>>> + struct prb_data_ring *data_ring, > >>>> + unsigned long lpos_begin, > >>>> + unsigned long lpos_end, > >>>> + unsigned long *lpos_out) > >>>> +{ > >>>> + struct prb_desc_ring *desc_ring = &rb->desc_ring; > >>>> + struct prb_data_blk_lpos *blk_lpos; > >>>> + struct prb_data_block *blk; > >>>> + unsigned long tail_lpos; > >>>> + enum desc_state d_state; > >>>> + struct prb_desc desc; > >>>> + unsigned long id; > >>>> + > >>>> + /* > >>>> + * Using the provided @data_ring, point @blk_lpos to the correct > >>>> + * blk_lpos within the local copy of the descriptor. > >>>> + */ > >>>> + if (data_ring == &rb->text_data_ring) > >>>> + blk_lpos = &desc.text_blk_lpos; > >>>> + else > >>>> + blk_lpos = &desc.dict_blk_lpos; > >>>> + > >>>> + /* Loop until @lpos_begin has advanced to or beyond @lpos_end. > >>>> */ > >>>> + while ((lpos_end - lpos_begin) - 1 < DATA_SIZE(data_ring)) { > >>>> + blk = to_block(data_ring, lpos_begin); > >>>> + id = READ_ONCE(blk->id); /* LMM(data_make_reusable:A) */ > >>>> + > >>>> + /* > >>>> + * Guarantee the block ID is loaded before checking the > >>>> tail > >>>> + * lpos. The loaded block ID can only be considered > >>>> valid if > >>>> + * the tail lpos has not overtaken @lpos_begin. This > >>>> pairs > >>>> + * with data_alloc:A. > >>>> + * > >>>> + * Memory barrier involvement: > >>>> + * > >>>> + * If data_make_reusable:A reads from data_alloc:B, then > >>>> + * data_make_reusable:C reads from data_push_tail:D. > >>>> + * > >>>> + * Relies on: > >>>> + * > >>>> + * MB from data_push_tail:D to data_alloc:B > >>>> + * matching > >>>> + * RMB from data_make_reusable:A to data_make_reusable:C > >>>> + * > >>>> + * Note: data_push_tail:D and data_alloc:B can be > >>>> different > >>>> + * CPUs. However, the data_alloc:B CPU (which > >>>> performs > >>>> + * the full memory barrier) must have previously > >>>> seen > >>>> + * data_push_tail:D. > >>>> + */ > >>>> + smp_rmb(); /* LMM(data_make_reusable:B) */ > >>>> + > >>>> + tail_lpos = atomic_long_read(&data_ring->tail_lpos > >>>> + ); /* LMM(data_make_reusable:C) > >>>> */ > >>>> + > >>>> + /* > >>>> + * If @lpos_begin has fallen behind the tail lpos, the > >>>> read > >>>> + * block ID cannot be trusted. Fast forward @lpos_begin > >>>> to the > >>>> + * tail lpos and try again. > >>>> + */ > >>>> + if (lpos_begin - tail_lpos >= DATA_SIZE(data_ring)) { > >>>> + lpos_begin = tail_lpos; > >>>> + continue; > >>>> + } > >>> > >>> I am sorry if we have had this discussion already in past. > >> > >> We have [0]. (Search for "Ouch.") > > > > I see. Thanks a lot for the pointer. > > > >>> Well, it would just prove that it really needs a comment why this > >>> check is necessary. > >> > >> The comment says why it is necessary. The previous read of the block ID > >> cannot be trusted if the the tail has been pushed beyond it. > > > > Not really. The comment describes what the check does. But it does not > > explain why it is needed. The reason might be described be something like: > > > > * Make sure that the id read from tail_lpos is really > > * pointing to this lpos. The block might have been > > * reused in the meantime. Make sure to do not make > > * the new owner reusable. > > That is _not_ what this check is doing. I recommend looking closely at > the example you posted. This is not about whether or not a descriptor is > pointing to this lpos. In your example you showed that ID, state, and > lpos values could all look good, but it is for the _new_ record and we > should _not_ invalidate that one.
OK, let's make sure that we are talking about the same example. I was talking about this one from https://lore.kernel.org/lkml/[email protected]/ % [*] Another problem would be when data_make_reusable() see the new % data already in the committed state. It would make fresh new % data reusable. % % I mean the following: % % CPU0 CPU1 % % data_alloc() % begin_lpos = dr->head_lpos % data_push_tail() % lpos = dr->tail_lpos % prb_reserve() % # reserve the location of current % # dr->tail_lpos % prb_commit() % % id = blk->id % # read id for the freshly written data on CPU1 % # and happily make them reusable % data_make_reusable() Sigh, sigh, sigh, there is a hugely misleading comment in the example: % # reserve the location of current % # dr->tail_lpos It is true that it reserves part of this location. But it will use data_ring->head_lpos for the related desc->text_blk_lpos.begin !!! See below: > We can detect the scenario you pointed out by verifying the tail has not > moved beyond the data block that the ID was read from. The comment for > this check says: > > If @lpos_begin has fallen behind the tail lpos, > the read block ID cannot be trusted. > > This is exactly the why. It is only about whether we can trust that a > non-garbage block ID was read. Or do you want me to add: > > ... because data read that is behind the tail lpos must be > considered garbage. > > > But wait! This situation should get caught by the two existing descriptor > > checks: > > > >> case desc_committed: > >> /* > >> * This data block is invalid if the descriptor > >> * does not point back to it. > >> */ > >> if (blk_lpos->begin != lpos_begin) > >> return false; > >> desc_make_reusable(desc_ring, id); > >> break; > >> case desc_reusable: > >> /* > >> * This data block is invalid if the descriptor > >> * does not point back to it. > >> */ > >> if (blk_lpos->begin != lpos_begin) > >> return false; > >> break; > > No. Your example showed that it is not caught here. I am afraid that my example was wrong: If blk->id comes from the new descriptor written by CPU1 then blk_lpos->begin is based on the old data_ring->head_lpos. Then it is different from lpos_begin. Let's put it another way. The state of the descriptor defines validity of the data. Descriptor in committed state _must not_ point to invalid data block!!! If a descriptor in committed state point to lpos that was in invalid range before reading the descriptor then we have a huge hole in the design. This is why I believe that the check of the descriptor must be enough. Best Regards, Petr PS: I am sorry if I create too much confusion. It is easy to get lost :-( _______________________________________________ kexec mailing list [email protected] http://lists.infradead.org/mailman/listinfo/kexec
