4.14-stable review patch. If anyone has any objections, please let me know.
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[ Upstream commit fd5f7cde1b85d4c8e09ca46ce948e008a2377f64 ]
This patch, basically, reverts commit 6b97a20d3a79 ("printk: set may_schedule for some of console_trylock() callers"). That commit was a mistake, it introduced a big dependency on the scheduler, by enabling preemption under console_sem in printk()->console_unlock() path, which is rather too critical. The patch did not significantly reduce the possibilities of printk() lockups, but made it possible to stall printk(), as has been reported by Tetsuo Handa [1].
Another issues is that preemption under console_sem also messes up with Steven Rostedt's hand off scheme, by making it possible to sleep with console_sem both in console_unlock() and in vprintk_emit(), after acquiring the console_sem ownership (anywhere between printk_safe_exit_irqrestore() in console_trylock_spinning() and printk_safe_enter_irqsave() in console_unlock()). This makes hand off less likely and, at the same time, may result in a significant amount of pending logbuf messages. Preempted console_sem owner makes it impossible for other CPUs to emit logbuf messages, but does not make it impossible for other CPUs to append new messages to the logbuf.
Reinstate the old behavior and make printk() non-preemptible. Should any printk() lockup reports arrive they must be handled in a different way.
[1] http://lkml.kernel.org/r/201603022101.CAH73907.OVOOMFHFFtQJSL%20()%20I-love%... Fixes: 6b97a20d3a79 ("printk: set may_schedule for some of console_trylock() callers") Link: http://lkml.kernel.org/r/20180116044716.GE6607@jagdpanzerIV To: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Cc: Sergey Senozhatsky sergey.senozhatsky@gmail.com Cc: Tejun Heo tj@kernel.org Cc: akpm@linux-foundation.org Cc: linux-mm@kvack.org Cc: Cong Wang xiyou.wangcong@gmail.com Cc: Dave Hansen dave.hansen@intel.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Mel Gorman mgorman@suse.de Cc: Michal Hocko mhocko@kernel.org Cc: Vlastimil Babka vbabka@suse.cz Cc: Peter Zijlstra peterz@infradead.org Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Jan Kara jack@suse.cz Cc: Mathieu Desnoyers mathieu.desnoyers@efficios.com Cc: Byungchul Park byungchul.park@lge.com Cc: Pavel Machek pavel@ucw.cz Cc: linux-kernel@vger.kernel.org Signed-off-by: Sergey Senozhatsky sergey.senozhatsky@gmail.com Reported-by: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Reviewed-by: Steven Rostedt (VMware) rostedt@goodmis.org Signed-off-by: Petr Mladek pmladek@suse.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/printk/printk.c | 22 ++++++++-------------- 1 file changed, 8 insertions(+), 14 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 2d1c2700bd85..2f654a79f80b 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1902,6 +1902,12 @@ asmlinkage int vprintk_emit(int facility, int level,
/* If called from the scheduler, we can not call up(). */ if (!in_sched) { + /* + * Disable preemption to avoid being preempted while holding + * console_sem which would prevent anyone from printing to + * console + */ + preempt_disable(); /* * Try to acquire and then immediately release the console * semaphore. The release will print out buffers and wake up @@ -1909,6 +1915,7 @@ asmlinkage int vprintk_emit(int facility, int level, */ if (console_trylock_spinning()) console_unlock(); + preempt_enable(); }
return printed_len; @@ -2225,20 +2232,7 @@ int console_trylock(void) return 0; } console_locked = 1; - /* - * When PREEMPT_COUNT disabled we can't reliably detect if it's - * safe to schedule (e.g. calling printk while holding a spin_lock), - * because preempt_disable()/preempt_enable() are just barriers there - * and preempt_count() is always 0. - * - * RCU read sections have a separate preemption counter when - * PREEMPT_RCU enabled thus we must take extra care and check - * rcu_preempt_depth(), otherwise RCU read sections modify - * preempt_count(). - */ - console_may_schedule = !oops_in_progress && - preemptible() && - !rcu_preempt_depth(); + console_may_schedule = 0; return 1; } EXPORT_SYMBOL(console_trylock);