Messages in this thread | | | From | John Ogness <> | Subject | Re: [PATCH printk v2 1/9] printk: ringbuffer: Do not skip non-finalized records with prb_next_seq() | Date | Tue, 21 Nov 2023 17:23:35 +0106 |
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On 2023-11-20, Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote: > based on my research this should be the most recent post of this patch. > If so then > > On 2023-11-06 22:13:22 [+0106], John Ogness wrote: >> --- a/kernel/printk/printk_ringbuffer.c >> +++ b/kernel/printk/printk_ringbuffer.c >> + /* >> + * The provided sequence is only the lower 32 bits of the ringbuffer >> + * sequence. It needs to be expanded to 64bit. Get the first sequence >> + * number from the ringbuffer and fold it. >> + */ >> + seq = rb_first_seq - ((u32)rb_first_seq - ulseq); > > This needs to become > seq = rb_first_seq - ((s32)((u32)rb_first_seq - ulseq)); > > in order to continue booting on 32bit.
Indeed. The code assumes the passed in value (@ulseq) always represents a 64-bit number that is less than or equal to the basis value (@rb_first_seq). For kernel/printk/nbcon.c:__nbcon_seq_to_seq() that assumption is correct. For this function, it is not.
Your change will round up or down to the nearest 32 bits of the basis value.
For example, with @rb_first_seq = 0x200000000 and @ulseq = 0x1:
before your change (where @ulseq cannot represent something higher than @rb_first_seq):
@ulseq translates to 0x100000001
after your change:
@ulseq translates to 0x200000001
Since __ulseq_to_u64seq() must deal with arbitrary values, I think the 32-bit rounding is appropriate.
Despite not strictly being necessary (because of the valid assumption), I think we should also update __nbcon_seq_to_seq() to avoid any bizarre cases due to different translations of the 32-bit value.
In fact, there is no reason to have 2 macros for this. I will create a single macro using the 32-bit rounding.
Thanks for researching this!
John
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