From: Thomas Gleixner <tglx@linutronix.de> To: Bart Van Assche <Bart.VanAssche@wdc.com> Cc: "mingo@kernel.org" <mingo@kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "peterz@infradead.org" <peterz@infradead.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "byungchul.park@lge.com" <byungchul.park@lge.com>, "kernel-team@lge.com" <kernel-team@lge.com> Subject: Re: [PATCH v2 2/3] lockdep: Remove BROKEN flag of LOCKDEP_CROSSRELEASE Date: Thu, 19 Oct 2017 21:04:09 +0200 (CEST) [thread overview] Message-ID: <alpine.DEB.2.20.1710192021480.2054@nanos> (raw) In-Reply-To: <1508428021.2429.22.camel@wdc.com> Bart, On Thu, 19 Oct 2017, Bart Van Assche wrote: > It seems like you are missing my point. That might be a perception problem. > Cross-release checking is really *broken* as a concept. It is impossible > to improve it to the same reliability level as the kernel v4.13 lockdep > code. Hence my request to make it possible to disable cross-release > checking if PROVE_LOCKING is enabled. I did not read it as a request. If you'd had said: I have doubts about the concept and I think that it's impossible to handle the false positives up to the point where the existing lockdep infrastructure can do. Therefore I request that the feature gets an extra Kconfig entry (default y) or a command line parameter which allows to disable it in case of hard to fix false positive warnings, so the issue can be reported and normal lockdep testing can be resumed until the issue is fixed. Then I would have said: That makes sense, as long as its default on and people actually report the problems so the responsible developers can tackle them. What tripped me over was your statement: Many kernel developers, including myself, are not interested in spending time on analyzing false positive deadlock reports. Which sends a completely different message. > Consider the following example from the cross-release documentation: > > TASK X TASK Y > ------ ------ > acquire AX > acquire B /* A dependency 'AX -> B' exists */ > release B > release AX held by Y > > My understanding is that the cross-release code will add (AX, B) to the lock > order graph after having encountered the above code. I think that's wrong > because if the following sequence (Y: acquire AX, X: acquire B, X: release B) > is encountered again that there is no guarantee that AX can only be released > by X. Any task other than X could release that synchronization object too. Emphasis on could. That's not a lockdep problem and neither can the pure locking dependency tracking know that a particular deadlock is not possible by design. It can merily record the dependency chains and detect circular dependencies. There is enough code which is obviously correct in terms of locking which has lockdep annotations in one form or the other (nesting, different lock_class_keys etc.). These annotations are there to teach lockdep about false positives. It's pretty much the same with the cross release feature and we won't get these annotations into the code when people disable it Thanks, tglx
WARNING: multiple messages have this Message-ID (diff)
From: Thomas Gleixner <tglx@linutronix.de> To: Bart Van Assche <Bart.VanAssche@wdc.com> Cc: "mingo@kernel.org" <mingo@kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "peterz@infradead.org" <peterz@infradead.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "byungchul.park@lge.com" <byungchul.park@lge.com>, "kernel-team@lge.com" <kernel-team@lge.com> Subject: Re: [PATCH v2 2/3] lockdep: Remove BROKEN flag of LOCKDEP_CROSSRELEASE Date: Thu, 19 Oct 2017 21:04:09 +0200 (CEST) [thread overview] Message-ID: <alpine.DEB.2.20.1710192021480.2054@nanos> (raw) In-Reply-To: <1508428021.2429.22.camel@wdc.com> Bart, On Thu, 19 Oct 2017, Bart Van Assche wrote: > It seems like you are missing my point. That might be a perception problem. > Cross-release checking is really *broken* as a concept. It is impossible > to improve it to the same reliability level as the kernel v4.13 lockdep > code. Hence my request to make it possible to disable cross-release > checking if PROVE_LOCKING is enabled. I did not read it as a request. If you'd had said: I have doubts about the concept and I think that it's impossible to handle the false positives up to the point where the existing lockdep infrastructure can do. Therefore I request that the feature gets an extra Kconfig entry (default y) or a command line parameter which allows to disable it in case of hard to fix false positive warnings, so the issue can be reported and normal lockdep testing can be resumed until the issue is fixed. Then I would have said: That makes sense, as long as its default on and people actually report the problems so the responsible developers can tackle them. What tripped me over was your statement: Many kernel developers, including myself, are not interested in spending time on analyzing false positive deadlock reports. Which sends a completely different message. > Consider the following example from the cross-release documentation: > > TASK X TASK Y > ------ ------ > acquire AX > acquire B /* A dependency 'AX -> B' exists */ > release B > release AX held by Y > > My understanding is that the cross-release code will add (AX, B) to the lock > order graph after having encountered the above code. I think that's wrong > because if the following sequence (Y: acquire AX, X: acquire B, X: release B) > is encountered again that there is no guarantee that AX can only be released > by X. Any task other than X could release that synchronization object too. Emphasis on could. That's not a lockdep problem and neither can the pure locking dependency tracking know that a particular deadlock is not possible by design. It can merily record the dependency chains and detect circular dependencies. There is enough code which is obviously correct in terms of locking which has lockdep annotations in one form or the other (nesting, different lock_class_keys etc.). These annotations are there to teach lockdep about false positives. It's pretty much the same with the cross release feature and we won't get these annotations into the code when people disable it Thanks, tglx -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2017-10-19 19:04 UTC|newest] Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-10-19 5:55 [PATCH v2 0/3] crossrelease: make it not unwind by default Byungchul Park 2017-10-19 5:55 ` Byungchul Park 2017-10-19 5:55 ` [PATCH v2 1/3] lockdep: Introduce CROSSRELEASE_STACK_TRACE and make it not unwind as default Byungchul Park 2017-10-19 5:55 ` Byungchul Park 2017-10-19 5:55 ` [PATCH v2 2/3] lockdep: Remove BROKEN flag of LOCKDEP_CROSSRELEASE Byungchul Park 2017-10-19 5:55 ` Byungchul Park 2017-10-19 15:05 ` Bart Van Assche 2017-10-19 15:05 ` Bart Van Assche 2017-10-19 15:34 ` Thomas Gleixner 2017-10-19 15:34 ` Thomas Gleixner 2017-10-19 15:47 ` Bart Van Assche 2017-10-19 19:04 ` Thomas Gleixner [this message] 2017-10-19 19:04 ` Thomas Gleixner 2017-10-19 19:12 ` Thomas Gleixner 2017-10-19 19:12 ` Thomas Gleixner 2017-10-19 20:21 ` Bart Van Assche 2017-10-19 20:21 ` Bart Van Assche 2017-10-19 20:33 ` Matthew Wilcox 2017-10-19 20:33 ` Matthew Wilcox 2017-10-19 20:41 ` Bart Van Assche 2017-10-19 20:53 ` Thomas Gleixner 2017-10-19 20:53 ` Thomas Gleixner 2017-10-19 20:49 ` Thomas Gleixner 2017-10-19 20:49 ` Thomas Gleixner 2017-10-20 7:30 ` Ingo Molnar 2017-10-20 7:30 ` Ingo Molnar 2017-10-20 6:03 ` Byungchul Park 2017-10-20 6:03 ` Byungchul Park 2017-10-19 5:55 ` [PATCH v2 3/3] lockdep: Add a kernel parameter, crossrelease_fullstack Byungchul Park 2017-10-19 5:55 ` Byungchul Park 2017-10-19 7:03 ` [PATCH v2 0/4] Fix false positives by cross-release feature Byungchul Park 2017-10-19 7:03 ` Byungchul Park 2017-10-19 7:03 ` [PATCH v2 1/4] completion: Add support for initializing completion with lockdep_map Byungchul Park 2017-10-19 7:03 ` Byungchul Park 2017-10-19 7:03 ` [PATCH v2 2/4] lockdep: Remove unnecessary acquisitions wrt workqueue flush Byungchul Park 2017-10-19 7:03 ` Byungchul Park 2017-10-19 7:03 ` [PATCH v2 3/4] genhd.h: Remove trailing white space Byungchul Park 2017-10-19 7:03 ` Byungchul Park 2017-10-19 7:03 ` [PATCH v2 4/4] lockdep: Assign a lock_class per gendisk used for wait_for_completion() Byungchul Park 2017-10-19 7:03 ` Byungchul Park 2017-10-20 14:44 ` Christoph Hellwig 2017-10-20 14:44 ` Christoph Hellwig 2017-10-20 14:44 ` Christoph Hellwig 2017-10-22 23:53 ` Byungchul Park 2017-10-22 23:53 ` Byungchul Park 2017-10-23 6:36 ` Christoph Hellwig 2017-10-23 6:36 ` Christoph Hellwig 2017-10-23 7:04 ` Byungchul Park 2017-10-23 7:04 ` Byungchul Park 2017-10-21 19:17 ` kbuild test robot
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