lkml.org 
[lkml]   [2021]   [Jun]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    SubjectRe: [PATCH] printk/console: Check consistent sequence number when handling race in console_unlock()
    Date
    On 2021-06-29, Petr Mladek <pmladek@suse.com> wrote:
    > The standard printk() tries to flush the message to the console
    > immediately. It tries to take the console lock. If the lock is
    > already taken then the current owner is responsible for flushing
    > even the new message.
    >
    > There is a small race window between checking whether a new message is
    > available and releasing the console lock. It is solved by re-checking
    > the state after releasing the console lock. If the check is positive
    > then console_unlock() tries to take the lock again and process the new
    > message as well.
    >
    > The commit 996e966640ddea7b535c ("printk: remove logbuf_lock") causes that
    > console_seq is not longer read atomically. As a result, the re-check might
    > be done with an inconsistent 64-bit index.
    >
    > Solve it by using the last sequence number that has been checked under
    > the console lock. In the worst case, it will take the lock again only
    > to realized that the new message has already been proceed. But it
    > was possible even before.
    >
    > Fixes: commit 996e966640ddea7b535c ("printk: remove logbuf_lock")
    > Cc: stable@vger.kernel.org # 5.13
    > Signed-off-by: Petr Mladek <pmladek@suse.com>

    Reviewed-by: John Ogness <john.ogness@linutronix.de>

    Good catch.

    John

    > ---
    > kernel/printk/printk.c | 7 +++++--
    > 1 file changed, 5 insertions(+), 2 deletions(-)
    >
    > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
    > index 142a58d124d9..87411084075e 100644
    > --- a/kernel/printk/printk.c
    > +++ b/kernel/printk/printk.c
    > @@ -2545,6 +2545,7 @@ void console_unlock(void)
    > bool do_cond_resched, retry;
    > struct printk_info info;
    > struct printk_record r;
    > + u64 next_seq;
    >
    > if (console_suspended) {
    > up_console_sem();
    > @@ -2654,8 +2655,10 @@ void console_unlock(void)
    > cond_resched();
    > }
    >
    > - console_locked = 0;
    > + /* Get consistent value of the next-to-be-used sequence number. */
    > + next_seq = console_seq;
    >
    > + console_locked = 0;
    > up_console_sem();
    >
    > /*
    > @@ -2664,7 +2667,7 @@ void console_unlock(void)
    > * there's a new owner and the console_unlock() from them will do the
    > * flush, no worries.
    > */
    > - retry = prb_read_valid(prb, console_seq, NULL);
    > + retry = prb_read_valid(prb, next_seq, NULL);
    > printk_safe_exit_irqrestore(flags);
    >
    > if (retry && console_trylock())
    > --
    > 2.26.2

    \
     
     \ /
      Last update: 2021-06-29 16:49    [W:2.526 / U:0.220 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site