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n_tty: Fix read_buf race condition, increment read_head after pushing data

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Commit 19e2ad6a09 ("n_tty: Remove overflow
tests from receive_buf() path") moved the increment of read_head into
the arguments list of read_buf_addr(). Function calls represent a
sequence point in C. Therefore read_head is incremented before the
character c is placed in the buffer. Since the circular read buffer is
a lock-less design since commit 6d76bd2618
("n_tty: Make N_TTY ldisc receive path lockless"), this creates a race
condition that leads to communication errors.

This patch modifies the code to increment read_head _after_ the data
is placed in the buffer and thus fixes the race for non-SMP machines.
To fix the problem for SMP machines, memory barriers must be added in
a separate patch.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Cc: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Christian Riesch 2014-11-13 05:53:26 +01:00 committed by Greg Kroah-Hartman
parent 8ad3b13526
commit 8bfbe2de76
1 changed files with 2 additions and 1 deletions
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3
drivers/tty/n_tty.c
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@ -321,7 +321,8 @@ static void n_tty_check_unthrottle(struct tty_struct *tty)
static inline void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
{
*read_buf_addr(ldata, ldata->read_head++) = c;
*read_buf_addr(ldata, ldata->read_head) = c;
ldata->read_head++;
}
/**