John, ----- Ursprüngliche Mail ----- > Von: "John Ogness" <[email protected]> > An: "Petr Mladek" <[email protected]> > CC: "Sergey Senozhatsky" <[email protected]>, "Sergey > Senozhatsky" <[email protected]>, > "Steven Rostedt" <[email protected]>, "tglx" <[email protected]>, > "linux-kernel" <[email protected]>, > "Thomas Meyer" <[email protected]>, "richard" <[email protected]> > Gesendet: Montag, 1. Februar 2021 17:51:14 > Betreff: Re: [PATCH printk-rework 09/12] um: synchronize kmsg_dumper
> (Added CC: Thomas Meyer, Richard Weinberger) > > On 2021-02-01, Petr Mladek <[email protected]> wrote: >>> In preparation for removing printk's @logbuf_lock, dumpers that have >>> assumed to be protected against parallel calls must provide their own >>> synchronization. Add a locally static spinlock to synchronize the >>> kmsg_dump call and temporary buffer usage. >>> >>> Signed-off-by: John Ogness <[email protected]> >>> --- >>> arch/um/kernel/kmsg_dump.c | 8 ++++++++ >>> 1 file changed, 8 insertions(+) >>> >>> diff --git a/arch/um/kernel/kmsg_dump.c b/arch/um/kernel/kmsg_dump.c >>> index f38349ad00ea..173999422ed8 100644 >>> --- a/arch/um/kernel/kmsg_dump.c >>> +++ b/arch/um/kernel/kmsg_dump.c >>> @@ -1,5 +1,6 @@ >>> // SPDX-License-Identifier: GPL-2.0 >>> #include <linux/kmsg_dump.h> >>> +#include <linux/spinlock.h> >>> #include <linux/console.h> >>> #include <shared/init.h> >>> #include <shared/kern.h> >>> @@ -9,8 +10,10 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper, >>> enum kmsg_dump_reason reason, >>> struct kmsg_dumper_iter *iter) >>> { >>> + static DEFINE_SPINLOCK(lock); >>> static char line[1024]; >>> struct console *con; >>> + unsigned long flags; >>> size_t len = 0; >>> >>> /* only dump kmsg when no console is available */ >>> @@ -25,11 +28,16 @@ static void kmsg_dumper_stdout(struct kmsg_dumper >>> *dumper, >>> if (con) >>> return; >>> >>> + if (!spin_trylock_irqsave(&lock, flags)) >>> + return; >>> + >>> printf("kmsg_dump:\n"); >>> while (kmsg_dump_get_line(iter, true, line, sizeof(line), &len)) { >>> line[len] = '\0'; >>> printf("%s", line); >>> } >>> + >>> + spin_unlock_irqrestore(&lock, flags); >> >> What exactly is synchronized here, please? >> Access to @line buffer or @iter or both? > > @line is being synchronized. @iter does not require synchronization. > >> It looks to me that the access to @line buffer was not synchronized >> before. kmsg_dump_get_line() used a lock internally but >> it was not synchronized with the later printf("%s", line); > > The line was previously synchronized for the kmsg_dump_get_line() > call. But yes, it was not synchronized after the call, which is a bug if > the dump is triggered on multiple CPUs simultaneously. The commit > message should also mention that it is handling that bug. > >> IMHO, this patch is not needed. > > I am not familiar enough with ARCH=um to know if dumps can be triggered > on multiple CPUs simultaneously. Perhaps ThomasM or Richard can chime in > here. Well, uml has no SMP support, so no parallel dumps. :-) Thanks, //richard
