From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> To: Petr Mladek <pmladek@suse.com> Cc: Steven Rostedt <rostedt@goodmis.org>, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, Peter Zijlstra <peterz@infradead.org>, Andrew Morton <akpm@linux-foundation.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Jiri Slaby <jslaby@suse.cz>, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] printk: Correctly handle preemption in console_unlock() Date: Wed, 18 Jan 2017 14:45:51 +0900 [thread overview] Message-ID: <20170118054551.GA881@jagdpanzerIV.localdomain> (raw) In-Reply-To: <20170116110012.GE20462@pathway.suse.cz> On (01/16/17 12:00), Petr Mladek wrote: [..] > > Makes perfect sense to me. The only thing that worries me is that it > > does change the logic slightly, and I'm not sure if this will have any > > ramifications with it. That is, console_unlock() use to always leave > > with console_may_schedule equal to zero, where console_unlock() clears > > it. With this change, console_unlock() no longer clears that variable. > > Will that have any side effects that we are unaware of? > > Good question! it does look a bit worrisome. > If I get it correctly, the variable should never be used without the > console semaphore. IMHO, if it was used without the semaphore or if > it was not set correctly when the semaphore was taken, it would be a > bug. It means that leaving the variable set might actually help > to find a buggy usage if there is any. > > My findings: > > + console_may_lock is set only by functions that get the console > semaphore. > > + The function that takes the semaphore and does not set the > variable is resume_console(). IMHO, it is a bug. > > We are on the safe side because the function is called from > the same context as suspend_console() and it allows rescheduling. > > > + I am not aware of any use of the variable without the > semaphore. But it is not easy to prove just be reading > the code. there is a function that clears @console_may_schedule out of console_sem scope - console_flush_on_panic(). so I *may be* can think about a worst case scenario of race condition between console_flush_on_panic()->console_may_schedule = 0 on panic CPU and console_unlock()->console_may_schedule = 1 from CPU that panic CPU failed to stop (smp_send_stop() can return with secondary CPUs still being online). thoughts? -ss
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From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> To: Petr Mladek <pmladek@suse.com> Cc: Steven Rostedt <rostedt@goodmis.org>, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, Peter Zijlstra <peterz@infradead.org>, Andrew Morton <akpm@linux-foundation.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Jiri Slaby <jslaby@suse.cz>, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] printk: Correctly handle preemption in console_unlock() Date: Wed, 18 Jan 2017 05:45:51 +0000 [thread overview] Message-ID: <20170118054551.GA881@jagdpanzerIV.localdomain> (raw) In-Reply-To: <20170116110012.GE20462@pathway.suse.cz> On (01/16/17 12:00), Petr Mladek wrote: [..] > > Makes perfect sense to me. The only thing that worries me is that it > > does change the logic slightly, and I'm not sure if this will have any > > ramifications with it. That is, console_unlock() use to always leave > > with console_may_schedule equal to zero, where console_unlock() clears > > it. With this change, console_unlock() no longer clears that variable. > > Will that have any side effects that we are unaware of? > > Good question! it does look a bit worrisome. > If I get it correctly, the variable should never be used without the > console semaphore. IMHO, if it was used without the semaphore or if > it was not set correctly when the semaphore was taken, it would be a > bug. It means that leaving the variable set might actually help > to find a buggy usage if there is any. > > My findings: > > + console_may_lock is set only by functions that get the console > semaphore. > > + The function that takes the semaphore and does not set the > variable is resume_console(). IMHO, it is a bug. > > We are on the safe side because the function is called from > the same context as suspend_console() and it allows rescheduling. > > > + I am not aware of any use of the variable without the > semaphore. But it is not easy to prove just be reading > the code. there is a function that clears @console_may_schedule out of console_sem scope - console_flush_on_panic(). so I *may be* can think about a worst case scenario of race condition between console_flush_on_panic()->console_may_schedule = 0 on panic CPU and console_unlock()->console_may_schedule = 1 from CPU that panic CPU failed to stop (smp_send_stop() can return with secondary CPUs still being online). thoughts? -ss
next prev parent reply other threads:[~2017-01-18 5:47 UTC|newest] Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-01-13 13:15 [PATCH] printk: Correctly handle preemption in console_unlock() Petr Mladek 2017-01-13 13:15 ` Petr Mladek 2017-01-13 16:05 ` Steven Rostedt 2017-01-13 16:05 ` Steven Rostedt 2017-01-16 11:00 ` Petr Mladek 2017-01-16 11:00 ` Petr Mladek 2017-01-18 5:45 ` Sergey Senozhatsky [this message] 2017-01-18 5:45 ` Sergey Senozhatsky 2017-01-18 7:21 ` Sergey Senozhatsky 2017-01-18 7:21 ` Sergey Senozhatsky 2017-01-25 12:34 ` Petr Mladek 2017-01-25 12:34 ` Petr Mladek 2017-01-14 6:28 ` Sergey Senozhatsky 2017-01-14 6:28 ` Sergey Senozhatsky 2017-01-16 11:38 ` Petr Mladek 2017-01-16 11:38 ` Petr Mladek 2017-01-16 11:58 ` Sergey Senozhatsky 2017-01-16 11:58 ` Sergey Senozhatsky 2017-01-16 12:48 ` Petr Mladek 2017-01-16 12:48 ` Petr Mladek 2017-01-16 13:26 ` Sergey Senozhatsky 2017-01-16 13:26 ` Sergey Senozhatsky 2017-01-16 13:43 ` Sergey Senozhatsky 2017-01-16 13:43 ` Sergey Senozhatsky 2017-01-16 14:14 ` Petr Mladek 2017-01-16 14:14 ` Petr Mladek 2017-01-16 15:19 ` Sergey Senozhatsky 2017-01-16 15:19 ` Sergey Senozhatsky 2017-01-16 15:43 ` Sergey Senozhatsky 2017-01-16 15:43 ` Sergey Senozhatsky 2017-01-16 16:35 ` Petr Mladek 2017-01-16 16:35 ` Petr Mladek 2017-01-16 13:41 ` Tetsuo Handa 2017-01-16 13:41 ` Tetsuo Handa
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