kernel/linux.git/include/linux/semaphore.h, branch v6.19.11

hung_task: show the blocker task if the task is hung on semaphore

2025-05-12T00:54:08+00:00

Inspired by mutex blocker tracking[1], this patch makes a trade-off to balance the overhead and utility of the hung task detector. Unlike mutexes, semaphores lack explicit ownership tracking, making it challenging to identify the root cause of hangs. To address this, we introduce a last_holder field to the semaphore structure, which is updated when a task successfully calls down() and cleared during up(). The assumption is that if a task is blocked on a semaphore, the holders must not have released it. While this does not guarantee that the last holder is one of the current blockers, it likely provides a practical hint for diagnosing semaphore-related stalls. With this change, the hung task detector can now show blocker task's info like below: [Tue Apr 8 12:19:07 2025] INFO: task cat:945 blocked for more than 120 seconds. [Tue Apr 8 12:19:07 2025] Tainted: G E 6.14.0-rc6+ #1 [Tue Apr 8 12:19:07 2025] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [Tue Apr 8 12:19:07 2025] task:cat state:D stack:0 pid:945 tgid:945 ppid:828 task_flags:0x400000 flags:0x00000000 [Tue Apr 8 12:19:07 2025] Call Trace: [Tue Apr 8 12:19:07 2025] [Tue Apr 8 12:19:07 2025] __schedule+0x491/0xbd0 [Tue Apr 8 12:19:07 2025] schedule+0x27/0xf0 [Tue Apr 8 12:19:07 2025] schedule_timeout+0xe3/0xf0 [Tue Apr 8 12:19:07 2025] ? __folio_mod_stat+0x2a/0x80 [Tue Apr 8 12:19:07 2025] ? set_ptes.constprop.0+0x27/0x90 [Tue Apr 8 12:19:07 2025] __down_common+0x155/0x280 [Tue Apr 8 12:19:07 2025] down+0x53/0x70 [Tue Apr 8 12:19:07 2025] read_dummy_semaphore+0x23/0x60 [Tue Apr 8 12:19:07 2025] full_proxy_read+0x5f/0xa0 [Tue Apr 8 12:19:07 2025] vfs_read+0xbc/0x350 [Tue Apr 8 12:19:07 2025] ? __count_memcg_events+0xa5/0x140 [Tue Apr 8 12:19:07 2025] ? count_memcg_events.constprop.0+0x1a/0x30 [Tue Apr 8 12:19:07 2025] ? handle_mm_fault+0x180/0x260 [Tue Apr 8 12:19:07 2025] ksys_read+0x66/0xe0 [Tue Apr 8 12:19:07 2025] do_syscall_64+0x51/0x120 [Tue Apr 8 12:19:07 2025] entry_SYSCALL_64_after_hwframe+0x76/0x7e [Tue Apr 8 12:19:07 2025] RIP: 0033:0x7f419478f46e [Tue Apr 8 12:19:07 2025] RSP: 002b:00007fff1c4d2668 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 [Tue Apr 8 12:19:07 2025] RAX: ffffffffffffffda RBX: 0000000000020000 RCX: 00007f419478f46e [Tue Apr 8 12:19:07 2025] RDX: 0000000000020000 RSI: 00007f4194683000 RDI: 0000000000000003 [Tue Apr 8 12:19:07 2025] RBP: 00007f4194683000 R08: 00007f4194682010 R09: 0000000000000000 [Tue Apr 8 12:19:07 2025] R10: fffffffffffffbc5 R11: 0000000000000246 R12: 0000000000000000 [Tue Apr 8 12:19:07 2025] R13: 0000000000000003 R14: 0000000000020000 R15: 0000000000020000 [Tue Apr 8 12:19:07 2025] [Tue Apr 8 12:19:07 2025] INFO: task cat:945 blocked on a semaphore likely last held by task cat:938 [Tue Apr 8 12:19:07 2025] task:cat state:S stack:0 pid:938 tgid:938 ppid:584 task_flags:0x400000 flags:0x00000000 [Tue Apr 8 12:19:07 2025] Call Trace: [Tue Apr 8 12:19:07 2025] [Tue Apr 8 12:19:07 2025] __schedule+0x491/0xbd0 [Tue Apr 8 12:19:07 2025] ? _raw_spin_unlock_irqrestore+0xe/0x40 [Tue Apr 8 12:19:07 2025] schedule+0x27/0xf0 [Tue Apr 8 12:19:07 2025] schedule_timeout+0x77/0xf0 [Tue Apr 8 12:19:07 2025] ? __pfx_process_timeout+0x10/0x10 [Tue Apr 8 12:19:07 2025] msleep_interruptible+0x49/0x60 [Tue Apr 8 12:19:07 2025] read_dummy_semaphore+0x2d/0x60 [Tue Apr 8 12:19:07 2025] full_proxy_read+0x5f/0xa0 [Tue Apr 8 12:19:07 2025] vfs_read+0xbc/0x350 [Tue Apr 8 12:19:07 2025] ? __count_memcg_events+0xa5/0x140 [Tue Apr 8 12:19:07 2025] ? count_memcg_events.constprop.0+0x1a/0x30 [Tue Apr 8 12:19:07 2025] ? handle_mm_fault+0x180/0x260 [Tue Apr 8 12:19:07 2025] ksys_read+0x66/0xe0 [Tue Apr 8 12:19:07 2025] do_syscall_64+0x51/0x120 [Tue Apr 8 12:19:07 2025] entry_SYSCALL_64_after_hwframe+0x76/0x7e [Tue Apr 8 12:19:07 2025] RIP: 0033:0x7f7c584a646e [Tue Apr 8 12:19:07 2025] RSP: 002b:00007ffdba8ce158 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 [Tue Apr 8 12:19:07 2025] RAX: ffffffffffffffda RBX: 0000000000020000 RCX: 00007f7c584a646e [Tue Apr 8 12:19:07 2025] RDX: 0000000000020000 RSI: 00007f7c5839a000 RDI: 0000000000000003 [Tue Apr 8 12:19:07 2025] RBP: 00007f7c5839a000 R08: 00007f7c58399010 R09: 0000000000000000 [Tue Apr 8 12:19:07 2025] R10: fffffffffffffbc5 R11: 0000000000000246 R12: 0000000000000000 [Tue Apr 8 12:19:07 2025] R13: 0000000000000003 R14: 0000000000020000 R15: 0000000000020000 [Tue Apr 8 12:19:07 2025] [1] https://lore.kernel.org/all/174046694331.2194069.15472952050240807469.stgit@mhiramat.tok.corp.google.com Link: https://lkml.kernel.org/r/20250414145945.84916-3-ioworker0@gmail.com Signed-off-by: Mingzhe Yang Signed-off-by: Lance Yang Suggested-by: Andrew Morton Suggested-by: Masami Hiramatsu (Google) Reviewed-by: Masami Hiramatsu (Google) Cc: Anna Schumaker Cc: Boqun Feng Cc: Ingo Molnar Cc: Joel Granados Cc: John Stultz Cc: Kent Overstreet Cc: Peter Zijlstra Cc: Sergey Senozhatsky Cc: Steven Rostedt Cc: Tomasz Figa Cc: Waiman Long Cc: Will Deacon Cc: Yongliang Gao Cc: Zi Li Signed-off-by: Andrew Morton

Change DEFINE_SEMAPHORE() to take a number argument

2023-04-18T18:15:24+00:00

Fundamentally semaphores are a counted primitive, but DEFINE_SEMAPHORE() does not expose this and explicitly creates a binary semaphore. Change DEFINE_SEMAPHORE() to take a number argument and use that in the few places that open-coded it using __SEMAPHORE_INITIALIZER(). Signed-off-by: Peter Zijlstra (Intel) [mcgrof: add some tribal knowledge about why some folks prefer binary sempahores over mutexes] Reviewed-by: Sergey Senozhatsky Reviewed-by: Davidlohr Bueso Signed-off-by: Luis Chamberlain

treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 436

2019-06-05T15:37:17+00:00

Based on 1 normalized pattern(s): distributed under the terms of the gnu gpl version 2 extracted by the scancode license scanner the SPDX license identifier GPL-2.0-only has been chosen to replace the boilerplate/reference in 2 file(s). Signed-off-by: Thomas Gleixner Reviewed-by: Armijn Hemel Reviewed-by: Allison Randal Cc: linux-spdx@vger.kernel.org Link: https://lkml.kernel.org/r/20190531190115.032570679@linutronix.de Signed-off-by: Greg Kroah-Hartman

locking/semaphore: Update the file path in documentation

2018-02-13T14:00:06+00:00

While reading this header I noticed that the locking stuff has moved to kernel/locking/*, so update the path in semaphore.h to point to that. Signed-off-by: Tycho Andersen Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/20180201114119.1090-1-tycho@tycho.ws Signed-off-by: Ingo Molnar

locking, semaphores: Annotate inner lock as raw

2011-09-13T09:11:57+00:00

There is no reason to have the spin_lock protecting the semaphore preemptible on -rt. Annotate it as a raw_spinlock. In mainline this change documents the low level nature of the lock - otherwise there's no functional difference. Lockdep and Sparse checking will work as usual. ( On rt this also solves lockdep complaining about the rt_mutex.wait_lock being not initialized. ) Signed-off-by: Thomas Gleixner Signed-off-by: Ingo Molnar

semaphore: Remove mutex emulation

2010-10-30T10:12:50+00:00

Semaphores used as mutexes have been deprecated for years. Now that all users are either converted to real semaphores or to mutexes remove the cruft. Signed-off-by: Thomas Gleixner Cc: Peter Zijlstra Cc: Christoph Hellwig LKML-Reference: <20100907125057.562399240@linutronix.de>

semaphore: Add DEFINE_SEMAPHORE

2010-09-08T13:04:10+00:00

The full cleanup of init_MUTEX[_LOCKED] and DECLARE_MUTEX has not been done. Some of the users are real semaphores and we should name them as such instead of confusing everyone with "MUTEX". Provide the infrastructure to get finally rid of init_MUTEX[_LOCKED] and DECLARE_MUTEX. Signed-off-by: Thomas Gleixner Cc: Peter Zijlstra Cc: Christoph Hellwig LKML-Reference: <20100907125054.795929962@linutronix.de>

Remove __DECLARE_SEMAPHORE_GENERIC

2008-07-24T12:31:21+00:00

There are no users of __DECLARE_SEMAPHORE_GENERIC in the kernel Signed-off-by: Matthew Wilcox

Improve semaphore documentation

2008-04-17T14:43:01+00:00

Move documentation from semaphore.h to semaphore.c as requested by Andrew Morton. Also reformat to kernel-doc style and add some more notes about the implementation. Signed-off-by: Matthew Wilcox

Simplify semaphore implementation

2008-04-17T14:42:54+00:00

By removing the negative values of 'count' and relying on the wait_list to indicate whether we have any waiters, we can simplify the implementation by removing the protection against an unlikely race condition. Thanks to David Howells for his suggestions. Signed-off-by: Matthew Wilcox