On Fri, Jan 22, 2021 at 09:25:44AM -0800, Doug Anderson wrote: > Hi, > > On Fri, Jan 22, 2021 at 3:06 AM Sumit Garg <sumit.garg@xxxxxxxxxx> wrote: > > > > Currently kdb uses in_interrupt() to determine whether its library > > code has been called from the kgdb trap handler or from a saner calling > > context such as driver init. This approach is broken because > > in_interrupt() alone isn't able to determine kgdb trap handler entry from > > normal task context. This can happen during normal use of basic features > > such as breakpoints and can also be trivially reproduced using: > > echo g > /proc/sysrq-trigger > > I guess an alternative to your patch is to fully eliminate GFP_KDB. > It always strikes me as a sub-optimal design to choose between > GFP_ATOMIC and GFP_KERNEL like this. Presumably others must agree > because otherwise I'd expect that the overall kernel would have > something like "GFP_AUTOMATIC"? > > It doesn't feel like it'd be that hard to do something more explicit. > From a quick glance: > > * I think kdb_defcmd() and kdb_defcmd2() are always called in response > to a user typing something on the kdb command line. Those should > always be GFP_ATOMIC, right? No. I'm afraid not. The kdb parser is also used to execute kernel/debug/kdb/kdb_cmds as part of the kdb initialization. This initialization happens from the init calls rather than from the kgdb trap handler code. When I first looked at Sumit's patch I had a similar reaction to you but, whilst it is clearly it's not impossible to pass flags into the kdb parser and all its subcommands, I concluded that GFP_KDB is a better approach. BTW the reason I insisted on getting rid of the in_atomic() was to make it clear that GFP_KDB discriminates between exactly two calling contexts (normal and kgdb trap handler). I was didn't want any hints that imply GFP_KDB is a (broken) implementation of something like GFP_AUTOMATIC! Daniel.
