On Wed, Apr 11, 2018 at 12:17:27PM -0300, Mauro Carvalho Chehab wrote:
> Em Wed, 11 Apr 2018 18:02:19 +0300
> Sakari Ailus <sakari.ailus@xxxxxx> escreveu:
>
> > On Wed, Apr 11, 2018 at 10:49:35AM -0300, Mauro Carvalho Chehab wrote:
> > > Em Wed, 11 Apr 2018 16:21:16 +0300
> > > Sakari Ailus <sakari.ailus@xxxxxx> escreveu:
> > >
> > >
> > > > > > > Btw, this is a very good reason why you should define the ioctl to
> > > > > > > have an integer argument instead of a struct with a __s32 field
> > > > > > > on it, as per my comment to patch 02/29:
> > > > > > >
> > > > > > > #define MEDIA_IOC_REQUEST_ALLOC _IOWR('|', 0x05, int)
> > > > > > >
> > > > > > > At 64 bit architectures, you're truncating the file descriptor!
> > > > > >
> > > > > > I'm not quite sure what do you mean. int is 32 bits on 64-bit systems as
> > > > > > well.
> > > > >
> > > > > Hmm.. you're right. I was thinking that it could be 64 bits on some
> > > > > archs like sparc64 (Tru64 C compiler declares it with 64 bits), but,
> > > > > according with:
> > > > >
> > > > > https://www.gnu.org/software/gnu-c-manual/gnu-c-manual.html
> > > > >
> > > > > This is not the case on gcc.
> > > >
> > > > Ok. The reasoning back then was that what "int" means varies across
> > > > compilers and languages. And the intent was to codify this to __s32 which
> > > > is what the kernel effectively uses.
> > >
> > > ...
> > >
> > > > The rest of the kernel uses int rather liberally in the uAPI so I'm not
> > > > sure in the end whether something desirable was achieved. Perhaps it'd be
> > > > good to go back to the original discussion to find out for sure.
> > > >
> > > > Still binaries compiled with Tru64 C compiler wouldn't work on Linux anyway
> > > > due to that difference.
> > > >
> > > > Well, I stop here for this begins to be off-topic. :-)
> > >
> > > Yes. Let's keep it as s32 as originally proposed. Just ignore my comments
> > > about that :-)
> > >
> > > > > > > > + get_task_comm(comm, current);
> > > > > > > > + snprintf(req->debug_str, sizeof(req->debug_str), "%s:%d",
> > > > > > > > + comm, fd);
> > > > > > >
> > > > > > > Not sure if it is a good idea to store the task that allocated
> > > > > > > the request. While it makes sense for the dev_dbg() below, it
> > > > > > > may not make sense anymore on other dev_dbg() you would be
> > > > > > > using it.
> > > > > >
> > > > > > The lifetime of the file handle roughly matches that of the request. It's
> > > > > > for debug only anyway.
> > > > > >
> > > > > > Better proposals are always welcome of course. But I think we should have
> > > > > > something here that helps debugging by meaningfully making the requests
> > > > > > identifiable from logs.
> > > > >
> > > > > What I meant to say is that one PID could be allocating the
> > > > > request, while some other one could be actually doing Q/DQ_BUF.
> > > > > On such scenario, the debug string could provide mislead prints.
> > > >
> > > > Um, yes, indeed it would no longer match the process. But the request is
> > > > still the same. That's actually a positive thing since it allows you to
> > > > identify the request.
> > > >
> > > > With a global ID space this was trivial; you could just print the request
> > > > ID and that was all that was ever needed. (I'm not proposing to consider
> > > > that though.)
> > > >
> > >
> > > IMO, a global ID number would work better than get_task_comm().
> > >
> > > Just add a static int monotonic counter and use it for the debug purposes,
> > > e. g.:
> > >
> > > {
> > > static unsigned int req_count = 0;
> > >
> > > snprintf(req->debug_str, sizeof(req->debug_str), "%u:%d",
> > > req_count++, fd);
> > >
> > > Ok, eventually, it will overflow, but, it will be unique within
> > > a reasonable timeframe to be good enough for debugging purposes.
> >
> > Yes, but you can't figure out which process allocated it anymore, making
> > associating kernel debug logs with user space process logs harder.
> >
> > How about process id + file handle? That still doesn't tell which process
> > operated on the request though, but I'm not sure whether that's really a
> > crucial piece of information.
>
> You don't need that. With dev_dbg() - and other *_dbg() macros - you can
> enable process ID for all debug messages.
With this, the problem again is that it does not uniquely identify the
request: the request is the same request independently of which process
would operate on it. Or whether it is being processed in an interrupt
context.
AFAICT, the allocator PID (or process name) + file handle are both required
to match a request between user and kernel space logs.
>
> Basically, if the user needs the PID, all it needs is to use "+pt",
> e. g. something like:
>
> echo "file drivers/media/* +pt" > /sys/kernel/debug/dynamic_debug/control
Thanks for the info. I didn't know this.
--
Regards,
Sakari Ailus
e-mail: sakari.ailus@xxxxxx
