On Wed, Apr 29, 2020 at 07:46:26AM +0000, Luis Chamberlain wrote: > We use one blktrace per request_queue, that means one per the entire > disk. So we cannot run one blktrace on say /dev/vda and then /dev/vda1, > or just two calls on /dev/vda. > > We check for concurrent setup only at the very end of the blktrace setup though. > > If we try to run two concurrent blktraces on the same block device the > second one will fail, and the first one seems to go on. However when > one tries to kill the first one one will see things like this: > > The kernel will show these: > > ``` > debugfs: File 'dropped' in directory 'nvme1n1' already present! > debugfs: File 'msg' in directory 'nvme1n1' already present! > debugfs: File 'trace0' in directory 'nvme1n1' already present! > `` > > And userspace just sees this error message for the second call: > > ``` > blktrace /dev/nvme1n1 > BLKTRACESETUP(2) /dev/nvme1n1 failed: 5/Input/output error > ``` > > The first userspace process #1 will also claim that the files > were taken underneath their nose as well. The files are taken > away form the first process given that when the second blktrace > fails, it will follow up with a BLKTRACESTOP and BLKTRACETEARDOWN. > This means that even if go-happy process #1 is waiting for blktrace > data, we *have* been asked to take teardown the blktrace. > > This can easily be reproduced with break-blktrace [0] run_0005.sh test. > > Just break out early if we know we're already going to fail, this will > prevent trying to create the files all over again, which we know still > exist. > > [0] https://github.com/mcgrof/break-blktrace > Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx> > --- > kernel/trace/blktrace.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c > index 5c52976bd762..383045f67cb8 100644 > --- a/kernel/trace/blktrace.c > +++ b/kernel/trace/blktrace.c > @@ -4,6 +4,8 @@ > * > */ > > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt > + > #include <linux/kernel.h> > #include <linux/blkdev.h> > #include <linux/blktrace_api.h> > @@ -516,6 +518,11 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev, > */ > strreplace(buts->name, '/', '_'); > > + if (q->blk_trace) { > + pr_warn("Concurrent blktraces are not allowed\n"); > + return -EBUSY; You have access to a block device here, please use dev_warn() instead here for that, that makes it obvious as to what device a "concurrent blktrace" was attempted for. thanks, greg k-h
