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Message-ID: <CAMuHMdUHFQcvHzYSQjXZduPUUiStS4Yyv45k5XNY2MeDjSjW0A@mail.gmail.com>
Date: Fri, 22 Aug 2025 09:37:00 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, zi.li@...ux.dev, anna.schumaker@...cle.com,
boqun.feng@...il.com, joel.granados@...nel.org, jstultz@...gle.com,
kent.overstreet@...ux.dev, leonylgao@...cent.com,
linux-kernel@...r.kernel.org, longman@...hat.com, mhiramat@...nel.org,
mingo@...hat.com, mingzhe.yang@...com, peterz@...radead.org,
rostedt@...dmis.org, senozhatsky@...omium.org, tfiga@...omium.org,
will@...nel.org, Lance Yang <lance.yang@...ux.dev>, Eero Tamminen <oak@...sinkinet.fi>,
linux-m68k <linux-m68k@...ts.linux-m68k.org>
Subject: Re: [PATCH 2/3] hung_task: extend hung task blocker tracking to rwsems
On Fri, 22 Aug 2025 at 09:34, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> On Fri, 27 Jun 2025 at 09:30, Lance Yang <ioworker0@...il.com> wrote:
> > From: Lance Yang <lance.yang@...ux.dev>
> >
> > Inspired by mutex blocker tracking[1], and having already extended it to
> > semaphores, let's now add support for reader-writer semaphores (rwsems).
> >
> > The approach is simple: when a task enters TASK_UNINTERRUPTIBLE while
> > waiting for an rwsem, we just call hung_task_set_blocker(). The hung task
> > detector can then query the rwsem's owner to identify the lock holder.
> >
> > Tracking works reliably for writers, as there can only be a single writer
> > holding the lock, and its task struct is stored in the owner field.
> >
> > The main challenge lies with readers. The owner field points to only one
> > of many concurrent readers, so we might lose track of the blocker if that
> > specific reader unlocks, even while others remain. This is not a
> > significant issue, however. In practice, long-lasting lock contention is
> > almost always caused by a writer. Therefore, reliably tracking the writer
> > is the primary goal of this patch series ;)
> >
> > With this change, the hung task detector can now show blocker task's info
> > like below:
> >
> > [Fri Jun 27 15:21:34 2025] INFO: task cat:28631 blocked for more than 122 seconds.
> > [Fri Jun 27 15:21:34 2025] Tainted: G S 6.16.0-rc3 #8
> > [Fri Jun 27 15:21:34 2025] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [Fri Jun 27 15:21:34 2025] task:cat state:D stack:0 pid:28631 tgid:28631 ppid:28501 task_flags:0x400000 flags:0x00004000
> > [Fri Jun 27 15:21:34 2025] Call Trace:
> > [Fri Jun 27 15:21:34 2025] <TASK>
> > [Fri Jun 27 15:21:34 2025] __schedule+0x7c7/0x1930
> > [Fri Jun 27 15:21:34 2025] ? __pfx___schedule+0x10/0x10
> > [Fri Jun 27 15:21:34 2025] ? policy_nodemask+0x215/0x340
> > [Fri Jun 27 15:21:34 2025] ? _raw_spin_lock_irq+0x8a/0xe0
> > [Fri Jun 27 15:21:34 2025] ? __pfx__raw_spin_lock_irq+0x10/0x10
> > [Fri Jun 27 15:21:34 2025] schedule+0x6a/0x180
> > [Fri Jun 27 15:21:34 2025] schedule_preempt_disabled+0x15/0x30
> > [Fri Jun 27 15:21:34 2025] rwsem_down_read_slowpath+0x55e/0xe10
> > [Fri Jun 27 15:21:34 2025] ? __pfx_rwsem_down_read_slowpath+0x10/0x10
> > [Fri Jun 27 15:21:34 2025] ? __pfx___might_resched+0x10/0x10
> > [Fri Jun 27 15:21:34 2025] down_read+0xc9/0x230
> > [Fri Jun 27 15:21:34 2025] ? __pfx_down_read+0x10/0x10
> > [Fri Jun 27 15:21:34 2025] ? __debugfs_file_get+0x14d/0x700
> > [Fri Jun 27 15:21:34 2025] ? __pfx___debugfs_file_get+0x10/0x10
> > [Fri Jun 27 15:21:34 2025] ? handle_pte_fault+0x52a/0x710
> > [Fri Jun 27 15:21:34 2025] ? selinux_file_permission+0x3a9/0x590
> > [Fri Jun 27 15:21:34 2025] read_dummy_rwsem_read+0x4a/0x90
> > [Fri Jun 27 15:21:34 2025] full_proxy_read+0xff/0x1c0
> > [Fri Jun 27 15:21:34 2025] ? rw_verify_area+0x6d/0x410
> > [Fri Jun 27 15:21:34 2025] vfs_read+0x177/0xa50
> > [Fri Jun 27 15:21:34 2025] ? __pfx_vfs_read+0x10/0x10
> > [Fri Jun 27 15:21:34 2025] ? fdget_pos+0x1cf/0x4c0
> > [Fri Jun 27 15:21:34 2025] ksys_read+0xfc/0x1d0
> > [Fri Jun 27 15:21:34 2025] ? __pfx_ksys_read+0x10/0x10
> > [Fri Jun 27 15:21:34 2025] do_syscall_64+0x66/0x2d0
> > [Fri Jun 27 15:21:34 2025] entry_SYSCALL_64_after_hwframe+0x76/0x7e
> > [Fri Jun 27 15:21:34 2025] RIP: 0033:0x7f3f8faefb40
> > [Fri Jun 27 15:21:34 2025] RSP: 002b:00007ffdeda5ab98 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> > [Fri Jun 27 15:21:34 2025] RAX: ffffffffffffffda RBX: 0000000000010000 RCX: 00007f3f8faefb40
> > [Fri Jun 27 15:21:34 2025] RDX: 0000000000010000 RSI: 00000000010fa000 RDI: 0000000000000003
> > [Fri Jun 27 15:21:34 2025] RBP: 00000000010fa000 R08: 0000000000000000 R09: 0000000000010fff
> > [Fri Jun 27 15:21:34 2025] R10: 00007ffdeda59fe0 R11: 0000000000000246 R12: 00000000010fa000
> > [Fri Jun 27 15:21:34 2025] R13: 0000000000000003 R14: 0000000000000000 R15: 0000000000000fff
> > [Fri Jun 27 15:21:34 2025] </TASK>
> > [Fri Jun 27 15:21:34 2025] INFO: task cat:28631 <reader> blocked on an rw-semaphore likely owned by task cat:28630 <writer>
> > [Fri Jun 27 15:21:34 2025] task:cat state:S stack:0 pid:28630 tgid:28630 ppid:28501 task_flags:0x400000 flags:0x00004000
> > [Fri Jun 27 15:21:34 2025] Call Trace:
> > [Fri Jun 27 15:21:34 2025] <TASK>
> > [Fri Jun 27 15:21:34 2025] __schedule+0x7c7/0x1930
> > [Fri Jun 27 15:21:34 2025] ? __pfx___schedule+0x10/0x10
> > [Fri Jun 27 15:21:34 2025] ? __mod_timer+0x304/0xa80
> > [Fri Jun 27 15:21:34 2025] schedule+0x6a/0x180
> > [Fri Jun 27 15:21:34 2025] schedule_timeout+0xfb/0x230
> > [Fri Jun 27 15:21:34 2025] ? __pfx_schedule_timeout+0x10/0x10
> > [Fri Jun 27 15:21:34 2025] ? __pfx_process_timeout+0x10/0x10
> > [Fri Jun 27 15:21:34 2025] ? down_write+0xc4/0x140
> > [Fri Jun 27 15:21:34 2025] msleep_interruptible+0xbe/0x150
> > [Fri Jun 27 15:21:34 2025] read_dummy_rwsem_write+0x54/0x90
> > [Fri Jun 27 15:21:34 2025] full_proxy_read+0xff/0x1c0
> > [Fri Jun 27 15:21:34 2025] ? rw_verify_area+0x6d/0x410
> > [Fri Jun 27 15:21:34 2025] vfs_read+0x177/0xa50
> > [Fri Jun 27 15:21:34 2025] ? __pfx_vfs_read+0x10/0x10
> > [Fri Jun 27 15:21:34 2025] ? fdget_pos+0x1cf/0x4c0
> > [Fri Jun 27 15:21:34 2025] ksys_read+0xfc/0x1d0
> > [Fri Jun 27 15:21:34 2025] ? __pfx_ksys_read+0x10/0x10
> > [Fri Jun 27 15:21:34 2025] do_syscall_64+0x66/0x2d0
> > [Fri Jun 27 15:21:34 2025] entry_SYSCALL_64_after_hwframe+0x76/0x7e
> > [Fri Jun 27 15:21:34 2025] RIP: 0033:0x7f8f288efb40
> > [Fri Jun 27 15:21:34 2025] RSP: 002b:00007ffffb631038 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> > [Fri Jun 27 15:21:34 2025] RAX: ffffffffffffffda RBX: 0000000000010000 RCX: 00007f8f288efb40
> > [Fri Jun 27 15:21:34 2025] RDX: 0000000000010000 RSI: 000000002a4b5000 RDI: 0000000000000003
> > [Fri Jun 27 15:21:34 2025] RBP: 000000002a4b5000 R08: 0000000000000000 R09: 0000000000010fff
> > [Fri Jun 27 15:21:34 2025] R10: 00007ffffb630460 R11: 0000000000000246 R12: 000000002a4b5000
> > [Fri Jun 27 15:21:34 2025] R13: 0000000000000003 R14: 0000000000000000 R15: 0000000000000fff
> > [Fri Jun 27 15:21:34 2025] </TASK>
> >
> > [1] https://lore.kernel.org/all/174046694331.2194069.15472952050240807469.stgit@mhiramat.tok.corp.google.com/
> >
> > Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > Suggested-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > Signed-off-by: Lance Yang <lance.yang@...ux.dev>
>
> Thanks for your patch, which is now commit 194a9b9e843b4077
> ("hung_task: show the blocker task if the task is hung on
> semaphore") in v6.16-rc1.
Sorry, wrong email thread.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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