From: Boqun Feng <boqun.feng@gmail.com>
To: Byungchul Park <byungchul.park@lge.com>
Cc: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
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Subject: Re: [PATCH RFC v7 00/23] DEPT(Dependency Tracker)
Date: Thu, 19 Jan 2023 19:07:59 -0800 [thread overview]
Message-ID: <Y8oFj9A19cw3enHB@boqun-archlinux> (raw)
In-Reply-To: <Y8n7NdFl9WEbGXH1@boqun-archlinux>
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;
next prev parent reply other threads:[~2023-01-20 3:08 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-09 3:33 [PATCH RFC v7 00/23] DEPT(Dependency Tracker) Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 01/23] llist: Move llist_{head,node} definition to types.h Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 02/23] dept: Implement Dept(Dependency Tracker) Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 03/23] dept: Add single event dependency tracker APIs Byungchul Park
2023-01-18 13:01 ` Thomas Gleixner
2023-01-09 3:33 ` [PATCH RFC v7 04/23] dept: Add lock " Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 05/23] dept: Tie to Lockdep and IRQ tracing Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 06/23] dept: Add proc knobs to show stats and dependency graph Byungchul Park
2023-01-18 12:56 ` Thomas Gleixner
2023-01-09 3:33 ` [PATCH RFC v7 07/23] dept: Apply sdt_might_sleep_strong() to wait_for_completion()/complete() Byungchul Park
2023-01-18 12:59 ` Thomas Gleixner
2023-01-09 3:33 ` [PATCH RFC v7 08/23] dept: Apply sdt_might_sleep_strong() to PG_{locked,writeback} wait Byungchul Park
2023-01-09 9:10 ` Sergey Shtylyov
2023-01-09 3:33 ` [PATCH RFC v7 09/23] dept: Apply sdt_might_sleep_weak() to swait Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 10/23] dept: Apply sdt_might_sleep_weak() to waitqueue wait Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 11/23] dept: Apply sdt_might_sleep_weak() to hashed-waitqueue wait Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 12/23] dept: Distinguish each syscall context from another Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 13/23] dept: Distinguish each work " Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 14/23] dept: Add a mechanism to refill the internal memory pools on running out Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 15/23] locking/lockdep, cpu/hotplus: Use a weaker annotation in AP thread Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 16/23] dept: Apply sdt_might_sleep_strong() to dma fence wait Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 17/23] dept: Track timeout waits separately with a new Kconfig Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 18/23] dept: Apply timeout consideration to wait_for_completion()/complete() Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 19/23] dept: Apply timeout consideration to swait Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 20/23] dept: Apply timeout consideration to waitqueue wait Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 21/23] dept: Apply timeout consideration to hashed-waitqueue wait Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 22/23] dept: Apply timeout consideration to dma fence wait Byungchul Park
2023-01-09 3:33 ` [PATCH RFC v7 23/23] dept: Record the latest one out of consecutive waits of the same class Byungchul Park
2023-01-16 18:00 ` [PATCH RFC v7 00/23] DEPT(Dependency Tracker) Linus Torvalds
2023-01-17 18:18 ` Boqun Feng
2023-01-17 18:40 ` Waiman Long
2023-01-18 12:55 ` Thomas Gleixner
2023-01-19 9:05 ` Byungchul Park
2023-01-19 6:23 ` Byungchul Park
2023-01-19 7:06 ` Byungchul Park
2023-01-19 13:33 ` Matthew Wilcox
2023-01-19 19:25 ` Boqun Feng
2023-01-20 1:51 ` Byungchul Park
2023-01-20 2:23 ` Boqun Feng
2023-01-20 3:07 ` Boqun Feng [this message]
2023-01-20 3:26 ` Boqun Feng
2023-01-21 3:28 ` Byungchul Park
2023-01-21 3:44 ` Boqun Feng
2023-01-21 4:01 ` Boqun Feng
2023-01-21 4:47 ` Byungchul Park
2023-01-19 0:58 ` Byungchul Park
2023-01-21 2:40 ` Byungchul Park
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