On Tue, Aug 14, 2012 at 9:05 AM, Seiji Aguchi <[email protected]> wrote: > Hi, > > I'm sending an email to discuss how to remove create_sysfs_entry() from a > write callback. > > [Problem] > > Current efi_pstore creates sysfs entries ,which enable users to access to > NVRAM, in a write callback. > If a kernel panic happens in interrupt contexts, pstore may fail because it > could sleep due to dynamic > memory allocations during creating sysfs entries. > > To resolve the problem above, my goal here is removing create_sysfs_entry() > from a write callback. > > [Ideas] > > (1) Introduce a workqueue updating sysfs entries > > To remove create_sysfs_entry() from a write callback, > It seems to be possible if efi_pstore updates its sysfs files > by scanning existing entries in NVRAM with a GetNextVariable() > in a workqueue. > > > I created a prototype patch based on an idea above but can't avoid a race > between SetVariable() in a write callback and GetNextVariable() in a > workqueue. > It is not guaranteed by EFI specification. > > EFI 2.3.1 specification, page 217. > <snip> > Calls to SetVariable() between calls to > GetNextVariableName() may produce unpredictable results. > <snip>
Can we not serialize this with &efivars->lock if it is updated to disable interrupts? A loop in the workqueue that locks, iterates through ->get_next_variable, and compares against searches in efivars->list should work, no? > > > (2) Don't support sysfs entries in efi_pstore. > > Another idea is _not_ updating sysfs entries at all in efi_pstore. > This can avoid a race SetVariable() and GetNextVariable(). > > write callback > - simply write a new entry with SetVariable(). > - This fits a discussion about holding multiple logs in a thread > below. > http://marc.info/?l=linux-kernel&m=134316268011854&w=2 > > erase callback > - simply erase an existing entry with SetVariable(). > > read callback > - Scaning existing entries with GetNextVariable(). > We can avoid a race between GetNextVariable() in a read callback > and SetVariable() in a write/erase callback by protecting them with > efi_lock. > > IMO, idea (2) is reasonable because we already have an interface, > /dev/pstore, which users can access > to NVRAM and we don't need to support multiple user interfaces. > > Any comments are welcome. > > Seiji -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
