On 11/25/21 21:12, Gerald Schaefer wrote: > On Thu, 25 Nov 2021 17:13:10 +0100 > Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx> wrote: > >> On Tue, 23 Nov 2021 15:19:49 +0100 >> Vlastimil Babka <vbabka@xxxxxxx> wrote: >> >>> On 11/22/21 21:33, Gerald Schaefer wrote: >>>> On Mon, 22 Nov 2021 21:14:00 +0100 >>>> Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx> wrote: >>>> >>>> [...] >>>>> >>>>> Thanks. While testing this properly, yet another bug showed up. The idx >>>>> in op->show remains 0 in all iterations, so I always see the same line >>>>> printed t->count times (or infinitely, ATM). Not sure if this only shows >>>>> on s390 due to endianness, but the reason is this: >>>>> >>>>> unsigned int idx = *(unsigned int *)v; >>> >>> Uh, good catch. I was actually looking suspiciously at how we cast signed to >>> unsigned, but didn't occur to me that shortening together with endiannes is >>> the problem. >>> >>>>> >>>>> IIUC, void *v is always the same as loff_t *ppos, and therefore idx also >>>>> should be *ppos. De-referencing the loff_t * with an unsigned int * only >>>>> gives the upper 32 bit half of the 64 bit value, which remains 0. >>>>> >>>>> This would be fixed e.g. with >>>>> >>>>> unsigned int idx = (unsigned int) *(loff_t *) v; >>> >>> With all this experience I'm now inclined to rather follow more the example >>> in Documentation/filesystems/seq_file.rst and don't pass around the pointer >>> that we got as ppos in slab_debugfs_start(), and that seq_file.c points to >>> m->index. >>> >>> In that example an own value is kmalloced: >>> >>> loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL); >>> >>> while we could just make this a field of loc_track? >> >> Yes, following the example sounds good, and it would also make proper use >> of *v in op->next, which might make the code more readable. It also looks >> like it already does exactly what is needed here, i.e. have a simple >> iterator that just counts the lines. >> >> I don't think the iterator needs to be saved in loc_track. IIUC, it is >> already passed around like in the example, and can then be simply compared >> to t->count, similar to the existing code. Saving it the loc_track doesn't preclude using the pointer that's being passed around. It however avoids the extra kmalloc and turns out it should also solve the return NULL from op->next() issue you describe below? >> This is what I'm currently testing, and it seems to work fine. Will send >> a new patch, if there are no objections: > > Oh well, I have one objection, returning NULL from op->next will be > passed to op->stop, and then it will not free the allocated value. > > The example is elegantly avoiding this, by not returning NULL anywhere, > and also not stopping. Sigh. > > Maybe not return NULL in op->next, but only from op->start, and only > when no allocation was made or it was freed already? Or free it only/ > already in op->next, when returning NULL? >From these two probably the "free in op->next". But still inclined to store it in loc_track. Why does the API need to be so awkward...
