linux-kernel - [PATCH 0/2] arm64: kgdb/kdb: Fix pending single-step debugging issues

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Message-Id: <20220411093819.1012583-1-sumit.garg@linaro.org>
Date:   Mon, 11 Apr 2022 15:08:17 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     linux-arm-kernel@...ts.infradead.org, dianders@...omium.org,
        will@...nel.org, liwei391@...wei.com
Cc:     catalin.marinas@....com, mark.rutland@....com, mhiramat@...nel.org,
        daniel.thompson@...aro.org, jason.wessel@...driver.com,
        linux-kernel@...r.kernel.org, Sumit Garg <sumit.garg@...aro.org>
Subject: [PATCH 0/2] arm64: kgdb/kdb: Fix pending single-step debugging issues

This patch-set reworks pending fixes from Wei's series [1] to make
single-step debugging via kgdb/kdb on arm64 work as expected. There was
a prior discussion on ML [2] regarding if we should keep the interrupts
enabled during single-stepping but it turns out that in case of kgdb, it
is risky to enable interrupts as sometimes a resume after single
stepping an interrupt handler leads to following unbalanced locking
issue:

[  300.328300] WARNING: bad unlock balance detected!
[  300.328608] 5.18.0-rc1-00016-g3e732ebf7316-dirty #6 Not tainted
[  300.329058] -------------------------------------
[  300.329298] sh/173 is trying to release lock (dbg_slave_lock) at:
[  300.329718] [<ffffd57c951c016c>] kgdb_cpu_enter+0x7ac/0x820
[  300.330029] but there are no more locks to release!
[  300.330265] 
[  300.330265] other info that might help us debug this:
[  300.330668] 4 locks held by sh/173:
[  300.330891]  #0: ffff4f5e454d8438 (sb_writers#3){.+.+}-{0:0}, at: vfs_write+0x98/0x204
[  300.331735]  #1: ffffd57c973bc2f0 (dbg_slave_lock){+.+.}-{2:2}, at: kgdb_cpu_enter+0x5b4/0x820
[  300.332259]  #2: ffffd57c973a9460 (rcu_read_lock){....}-{1:2}, at: kgdb_cpu_enter+0xe0/0x820
[  300.332717]  #3: ffffd57c973bc2a8 (dbg_master_lock){....}-{2:2}, at: kgdb_cpu_enter+0x1ec/0x820

So, I choose to keep interrupts disabled specifically for kgdb. This
series has been rebased to Linux 5.18-rc1 and I have dropped Doug's
review and test tags as there is significant rework involved.

[1] https://lore.kernel.org/all/20200509214159.19680-1-liwei391@huawei.com/
[2] https://lore.kernel.org/all/CAD=FV=Voyfq3Qz0T3RY+aYWYJ0utdH=P_AweB=13rcV8GDBeyQ@mail.gmail.com/

Sumit Garg (2):
  arm64: kgdb: Fix incorrect single stepping into the irq handler
  arm64: kgdb: Set PSTATE.SS to 1 to re-enable single-step

 arch/arm64/include/asm/debug-monitors.h |  1 +
 arch/arm64/kernel/debug-monitors.c      |  5 ++++
 arch/arm64/kernel/kgdb.c                | 35 +++++++++++++++++++++++--
 3 files changed, 39 insertions(+), 2 deletions(-)

-- 
2.25.1