Feedback-ID: iad51458e:Fastmail
Date:   Fri, 20 Jan 2023 19:44:01 -0800
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)
Message-ID: <Y8tfgYNZ//feEDvC@Boquns-Mac-mini.local>
References: <Y8oFj9A19cw3enHB@boqun-archlinux>
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On Sat, Jan 21, 2023 at 12:28:14PM +0900, Byungchul Park wrote:
> On Thu, Jan 19, 2023 at 07:07:59PM -0800, Boqun Feng wrote:
> > 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:
> 
> [...]
> 
> > > > 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.
> 
> I tried the 'queued read lock' test cases with DEPT on. I can see DEPT
> detect and report it. But yeah.. it's too verbose now. It's because DEPT
> is not aware of the test environment so it's just working hard to report
> every case.
> 
> To make DEPT work with the selftest better, some works are needed. I
> will work on it later or you please work on it.
> 
> The corresponding report is the following.
> 
[...]
> [    4.593037] context A's detail
> [    4.593351] ---------------------------------------------------
> [    4.593944] context A
> [    4.594182]     [S] lock(&rwlock_A:0)
> [    4.594577]     [W] lock(&rwlock_B:0)
> [    4.594952]     [E] unlock(&rwlock_A:0)
> [    4.595341] 
> [    4.595501] [S] lock(&rwlock_A:0):
> [    4.595848] [<ffffffff814eb244>] queued_read_lock_hardirq_ER_rE+0xf4/0x170
> [    4.596547] stacktrace:
> [    4.596797]       _raw_read_lock+0xcf/0x110
> [    4.597215]       queued_read_lock_hardirq_ER_rE+0xf4/0x170
> [    4.597766]       dotest+0x30/0x7bc
> [    4.598118]       locking_selftest+0x2c6f/0x2ead
> [    4.598602]       start_kernel+0x5aa/0x6d5
> [    4.599017]       secondary_startup_64_no_verify+0xe0/0xeb
> [    4.599562] 
[...]
> [    4.608427] [<ffffffff814eb3b4>] queued_read_lock_hardirq_RE_Er+0xf4/0x170
> [    4.609113] stacktrace:
> [    4.609366]       _raw_write_lock+0xc3/0xd0
> [    4.609788]       queued_read_lock_hardirq_RE_Er+0xf4/0x170
> [    4.610371]       dotest+0x30/0x7bc
> [    4.610730]       locking_selftest+0x2c41/0x2ead
> [    4.611195]       start_kernel+0x5aa/0x6d5
> [    4.611615]       secondary_startup_64_no_verify+0xe0/0xeb
> [    4.612164] 
> [    4.612325] [W] lock(&rwlock_A:0):
> [    4.612671] [<ffffffff814eb3c0>] queued_read_lock_hardirq_RE_Er+0x100/0x170
> [    4.613369] stacktrace:
> [    4.613622]       _raw_read_lock+0xac/0x110
> [    4.614047]       queued_read_lock_hardirq_RE_Er+0x100/0x170
> [    4.614652]       dotest+0x30/0x7bc
> [    4.615007]       locking_selftest+0x2c41/0x2ead
> [    4.615468]       start_kernel+0x5aa/0x6d5
> [    4.615879]       secondary_startup_64_no_verify+0xe0/0xeb
> [    4.616607] 
[...]

> As I told you, DEPT treats a queued lock as a normal type lock, no
> matter whether it's a read lock. That's why it prints just
> 'lock(&rwlock_A:0)' instead of 'read_lock(&rwlock_A:0)'. If needed, I'm
> gonna change the format.
> 
> I checked the selftest code and found, LOCK(B) is transformed like:
> 
> 	LOCK(B) -> WL(B) -> write_lock(&rwlock_B)
> 
> That's why '&rwlock_B' is printed instead of just 'B', JFYI.
> 

Nah, you output shows that you've run at least both function

	queued_read_lock_hardirq_RE_Er()
	queued_read_lock_hardirq_ER_rE()

but if you apply my diff

	https://lore.kernel.org/lkml/Y8oFj9A19cw3enHB@boqun-archlinux/

you should only run

	queued_read_lock_hardirq_RE_Er()

one test.

One of the reason that DEPT "detect" this is that DEPT doesn't reset
between tests, so old dependencies from previous run get carried over.


> Plus, for your information, you should turn on CONFIG_DEPT to use it.
> 

Yes I turn that config on.

Regards,
Boqun

> 	Byungchul