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Date: Sat, 21 Jan 2023 13:47:49 +0900
From: Byungchul Park <byungchul.park@....com>
To: boqun.feng@...il.com
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Subject: Re: [PATCH RFC v7 00/23] DEPT(Dependency Tracker)
Boqun wrote:
> 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()
Indeed! I'm sorry for that.
> 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.
I checked it. DEPT doesn't assume a rwlock switches between recursive
read lock and non-recursive read lock in a run time. Maybe it switches
since read lock needs to switch to recursive one in interrupt context.
By forcing read_lock_is_recursive() to always return false, DEPT works
as we expect. Otherwise, it doesn't.
Probabily I need to fix it.
Thanks.
Byungchul
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