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Thu, 19 Jan 2023 22:08:24 -0500 (EST) Date: Thu, 19 Jan 2023 19:07:59 -0800 From: Boqun Feng To: Byungchul Park Cc: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, damien.lemoal@opensource.wdc.com, linux-ide@vger.kernel.org, adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org, mingo@redhat.com, peterz@infradead.org, will@kernel.org, tglx@linutronix.de, rostedt@goodmis.org, joel@joelfernandes.org, sashal@kernel.org, daniel.vetter@ffwll.ch, duyuyang@gmail.com, johannes.berg@intel.com, tj@kernel.org, tytso@mit.edu, willy@infradead.org, david@fromorbit.com, amir73il@gmail.com, gregkh@linuxfoundation.org, kernel-team@lge.com, linux-mm@kvack.org, akpm@linux-foundation.org, mhocko@kernel.org, minchan@kernel.org, hannes@cmpxchg.org, vdavydov.dev@gmail.com, sj@kernel.org, jglisse@redhat.com, dennis@kernel.org, cl@linux.com, penberg@kernel.org, rientjes@google.com, vbabka@suse.cz, ngupta@vflare.org, linux-block@vger.kernel.org, paolo.valente@linaro.org, josef@toxicpanda.com, linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk, jack@suse.cz, jlayton@kernel.org, dan.j.williams@intel.com, hch@infradead.org, djwong@kernel.org, dri-devel@lists.freedesktop.org, rodrigosiqueiramelo@gmail.com, melissa.srw@gmail.com, hamohammed.sa@gmail.com, 42.hyeyoo@gmail.com, chris.p.wilson@intel.com, gwan-gyeong.mun@intel.com, max.byungchul.park@gmail.com, longman@redhat.com Subject: Re: [PATCH RFC v7 00/23] DEPT(Dependency Tracker) Message-ID: References: <1674179505-26987-1-git-send-email-byungchul.park@lge.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 6F44F140008 X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: 63gg1cgq48pebdey44ypor5k6gjhgrao X-HE-Tag: 1674184109-911368 X-HE-Meta: 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 vBGvZ8mR 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Jan 19, 2023 at 06:23:49PM -0800, Boqun Feng wrote: > On Fri, Jan 20, 2023 at 10:51:45AM +0900, Byungchul Park wrote: > > Boqun wrote: > > > On Thu, Jan 19, 2023 at 01:33:58PM +0000, Matthew Wilcox wrote: > > > > On Thu, Jan 19, 2023 at 03:23:08PM +0900, Byungchul Park wrote: > > > > > Boqun wrote: > > > > > > *Looks like the DEPT dependency graph doesn't handle the > > > > > > fair/unfair readers as lockdep current does. Which bring the > > > > > > next question. > > > > > > > > > > No. DEPT works better for unfair read. It works based on wait/event. So > > > > > read_lock() is considered a potential wait waiting on write_unlock() > > > > > while write_lock() is considered a potential wait waiting on either > > > > > write_unlock() or read_unlock(). DEPT is working perfect for it. > > > > > > > > > > For fair read (maybe you meant queued read lock), I think the case > > > > > should be handled in the same way as normal lock. I might get it wrong. > > > > > Please let me know if I miss something. > > > > > > > > From the lockdep/DEPT point of view, the question is whether: > > > > > > > > read_lock(A) > > > > read_lock(A) > > > > > > > > can deadlock if a writer comes in between the two acquisitions and > > > > sleeps waiting on A to be released. A fair lock will block new > > > > readers when a writer is waiting, while an unfair lock will allow > > > > new readers even while a writer is waiting. > > > > > > > > > > To be more accurate, a fair reader will wait if there is a writer > > > waiting for other reader (fair or not) to unlock, and an unfair reader > > > won't. > > > > What a kind guys, both of you! Thanks. > > > > I asked to check if there are other subtle things than this. Fortunately, > > I already understand what you guys shared. > > > > > In kernel there are read/write locks that can have both fair and unfair > > > readers (e.g. queued rwlock). Regarding deadlocks, > > > > > > T0 T1 T2 > > > -- -- -- > > > fair_read_lock(A); > > > write_lock(B); > > > write_lock(A); > > > write_lock(B); > > > unfair_read_lock(A); > > > > With the DEPT's point of view (let me re-write the scenario): > > > > T0 T1 T2 > > -- -- -- > > fair_read_lock(A); > > write_lock(B); > > write_lock(A); > > write_lock(B); > > unfair_read_lock(A); > > write_unlock(B); > > read_unlock(A); > > read_unlock(A); > > write_unlock(B); > > write_unlock(A); > > > > T0: read_unlock(A) cannot happen if write_lock(B) is stuck by a B owner > > not doing either write_unlock(B) or read_unlock(B). In other words: > > > > 1. read_unlock(A) happening depends on write_unlock(B) happening. > > 2. read_unlock(A) happening depends on read_unlock(B) happening. > > > > T1: write_unlock(B) cannot happen if unfair_read_lock(A) is stuck by a A > > owner not doing write_unlock(A). In other words: > > > > 3. write_unlock(B) happening depends on write_unlock(A) happening. > > > > 1, 2 and 3 give the following dependencies: > > > > 1. read_unlock(A) -> write_unlock(B) > > 2. read_unlock(A) -> read_unlock(B) > > 3. write_unlock(B) -> write_unlock(A) > > > > There's no circular dependency so it's safe. DEPT doesn't report this. > > > > > the above is not a deadlock, since T1's unfair reader can "steal" the > > > lock. However the following is a deadlock: > > > > > > T0 T1 T2 > > > -- -- -- > > > unfair_read_lock(A); > > > write_lock(B); > > > write_lock(A); > > > write_lock(B); > > > fair_read_lock(A); > > > > > > , since T'1 fair reader will wait. > > > > With the DEPT's point of view (let me re-write the scenario): > > > > T0 T1 T2 > > -- -- -- > > unfair_read_lock(A); > > write_lock(B); > > write_lock(A); > > write_lock(B); > > fair_read_lock(A); > > write_unlock(B); > > read_unlock(A); > > read_unlock(A); > > write_unlock(B); > > write_unlock(A); > > > > T0: read_unlock(A) cannot happen if write_lock(B) is stuck by a B owner > > not doing either write_unlock(B) or read_unlock(B). In other words: > > > > 1. read_unlock(A) happening depends on write_unlock(B) happening. > > 2. read_unlock(A) happening depends on read_unlock(B) happening. > > > > T1: write_unlock(B) cannot happen if fair_read_lock(A) is stuck by a A > > owner not doing either write_unlock(A) or read_unlock(A). In other > > words: > > > > 3. write_unlock(B) happening depends on write_unlock(A) happening. > > 4. write_unlock(B) happening depends on read_unlock(A) happening. > > > > 1, 2, 3 and 4 give the following dependencies: > > > > 1. read_unlock(A) -> write_unlock(B) > > 2. read_unlock(A) -> read_unlock(B) > > 3. write_unlock(B) -> write_unlock(A) > > 4. write_unlock(B) -> read_unlock(A) > > > > With 1 and 4, there's a circular dependency so DEPT definitely report > > this as a problem. > > > > REMIND: DEPT focuses on waits and events. > > Do you have the test cases showing DEPT can detect this? > Just tried the following on your latest GitHub branch, I commented all but one deadlock case. Lockdep CAN detect it but DEPT CANNOT detect it. Feel free to double check. Regards, Boqun ------------------------------------------->8 diff --git a/lib/locking-selftest.c b/lib/locking-selftest.c index cd89138d62ba..f38e4109e013 100644 --- a/lib/locking-selftest.c +++ b/lib/locking-selftest.c @@ -2375,6 +2375,7 @@ static void ww_tests(void) */ static void queued_read_lock_hardirq_RE_Er(void) { + // T0 HARDIRQ_ENTER(); read_lock(&rwlock_A); LOCK(B); @@ -2382,12 +2383,17 @@ static void queued_read_lock_hardirq_RE_Er(void) read_unlock(&rwlock_A); HARDIRQ_EXIT(); + // T1 HARDIRQ_DISABLE(); LOCK(B); read_lock(&rwlock_A); read_unlock(&rwlock_A); UNLOCK(B); HARDIRQ_ENABLE(); + + // T2 + write_lock_irq(&rwlock_A); + write_unlock_irq(&rwlock_A); } /* @@ -2455,6 +2461,7 @@ static void queued_read_lock_tests(void) dotest(queued_read_lock_hardirq_RE_Er, FAILURE, LOCKTYPE_RWLOCK); pr_cont("\n"); +#if 0 print_testname("hardirq lock-read/read-lock"); dotest(queued_read_lock_hardirq_ER_rE, SUCCESS, LOCKTYPE_RWLOCK); pr_cont("\n"); @@ -2462,6 +2469,7 @@ static void queued_read_lock_tests(void) print_testname("hardirq inversion"); dotest(queued_read_lock_hardirq_inversion, FAILURE, LOCKTYPE_RWLOCK); pr_cont("\n"); +#endif } static void fs_reclaim_correct_nesting(void) @@ -2885,6 +2893,7 @@ void locking_selftest(void) init_shared_classes(); lockdep_set_selftest_task(current); +#if 0 DO_TESTCASE_6R("A-A deadlock", AA); DO_TESTCASE_6R("A-B-B-A deadlock", ABBA); DO_TESTCASE_6R("A-B-B-C-C-A deadlock", ABBCCA); @@ -2967,6 +2976,7 @@ void locking_selftest(void) DO_TESTCASE_6x2x2RW("irq read-recursion #3", irq_read_recursion3); ww_tests(); +#endif force_read_lock_recursive = 0; /* @@ -2975,6 +2985,7 @@ void locking_selftest(void) if (IS_ENABLED(CONFIG_QUEUED_RWLOCKS)) queued_read_lock_tests(); +#if 0 fs_reclaim_tests(); /* Wait context test cases that are specific for RAW_LOCK_NESTING */ @@ -2987,6 +2998,7 @@ void locking_selftest(void) dotest(hardirq_deadlock_softirq_not_deadlock, FAILURE, LOCKTYPE_SPECIAL); pr_cont("\n"); +#endif if (unexpected_testcase_failures) { printk("-----------------------------------------------------------------\n"); debug_locks = 0;