c7c3f05e34
From printk()/serial console point of view panic() is special, because
it may force CPU to re-enter printk() or/and serial console driver.
Therefore, some of serial consoles drivers are re-entrant. E.g. 8250:
serial8250_console_write()
{
if (port->sysrq)
locked = 0;
else if (oops_in_progress)
locked = spin_trylock_irqsave(&port->lock, flags);
else
spin_lock_irqsave(&port->lock, flags);
...
}
panic() does set oops_in_progress via bust_spinlocks(1), so in theory
we should be able to re-enter serial console driver from panic():
CPU0
<NMI>
uart_console_write()
serial8250_console_write() // if (oops_in_progress)
// spin_trylock_irqsave()
call_console_drivers()
console_unlock()
console_flush_on_panic()
bust_spinlocks(1) // oops_in_progress++
panic()
<NMI/>
spin_lock_irqsave(&port->lock, flags) // spin_lock_irqsave()
serial8250_console_write()
call_console_drivers()
console_unlock()
printk()
...
However, this does not happen and we deadlock in serial console on
port->lock spinlock. And the problem is that console_flush_on_panic()
called after bust_spinlocks(0):
void panic(const char *fmt, ...)
{
bust_spinlocks(1);
...
bust_spinlocks(0);
console_flush_on_panic();
...
}
bust_spinlocks(0) decrements oops_in_progress, so oops_in_progress
can go back to zero. Thus even re-entrant console drivers will simply
spin on port->lock spinlock. Given that port->lock may already be
locked either by a stopped CPU, or by the very same CPU we execute
panic() on (for instance, NMI panic() on printing CPU) the system
deadlocks and does not reboot.
Fix this by removing bust_spinlocks(0), so oops_in_progress is always
set in panic() now and, thus, re-entrant console drivers will trylock
the port->lock instead of spinning on it forever, when we call them
from console_flush_on_panic().
Link: http://lkml.kernel.org/r/20181025101036.6823-1-sergey.senozhatsky@gmail.com
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Daniel Wang <wonderfly@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Peter Feiner <pfeiner@google.com>
Cc: linux-serial@vger.kernel.org
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
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| Documentation | ||
| LICENSES | ||
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| drivers | ||
| firmware | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
README
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.